OK then, time for me to sign off after an intriguing couple of days. Only one test remains, in Barcelona, and we will be back 08:00 GMT sharp on Thursday for live text coverage of all four days.
I'll be back in three weeks for the season opener in Australia.
Bring it on.
Get involved #bbcf1
Nick Bennetts: Only 3 weeks until Melbourne. Hurry up F1!
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MercedesCopyright: Mercedes
Mercedes: 131 laps for @nico_rosberg to cap off our week @Circuitcat_eng - top job everyone!
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So it is Romain Grosjean who finishes with the fastest time of the week, but that was set on supersoft tyres, meaning Nico Rosberg's 1:24.321 on medium tyres today was arguably the lap of the week, capping another impressive day for Mercedes.
Felipe Nasr had certainly been making up for lost time, having completed 70 laps this afternoon before that stop.
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Toro RossoCopyright: Toro Rosso
Toro Rosso: "Early finish for Carlos: a small mistake. But still a long, solid job: Laps 88, Best 1:25.604."
Red flag
Into the final few minutes and we have another red flag, with the Sauber of Felipe Nasr at a standstill at Turn Six.
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F1 journalist Dan Knutson in Barcelona: "After two hours and 45 minutes in the garage, Sebastian Vettel is back on track. He has the hard compound Pirellis on his Ferrari."
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LotusCopyright: Lotus
Lotus: 111 laps completed, fastest time of the day (so far), we'll call that a day.
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Romain Grosjean has the fastest lap of the day, will he look to improve on that? No, Lotus confirms his testing for today is done.
We've not seen this chap for a while - Sebastian Vettel. He finally re-emerges in the Ferrrari with 15 minutes.
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Nico Rosberg is back out on medium tyres and he shaves a couple of tenths off his previous best, clocking 1:24.321 to leave him just two tenths slower than Romain Grosjean's leading time.
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BBCCopyright: BBC
F1 journalist Dan Knutson in Barcelona: "The transporters will be staying put in the paddock as there are only three days between now and the next test here at the Circuit de Catalunya.
"The season starts with four flyaway races - Australia, Malaysia, China and Bahrain - so the next time the transporters will appear will be here in May at the Spanish Grand Prix which is the first race of the European season."
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Nico Rosberg is back out for the final 25 minutes of the second pre-season test. To highlight just how impressive that lap by Rosberg was, there is a 1.5-2secs difference between the medium and the supersoft.
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WilliamsCopyright: Williams
Williams: 122 laps so far as Valtteri Bottas heads off down the pitlane
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Romain Grosjean is back out in the Lotus but not for long as he makes a swift return to the pits. Valtteri Bottas heads back out as he looks to add to his impressive lap tally for today.
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WelshRacer: Rosberg time of 1m24.584 on medium compound is 9 tenths quicker than he did in 2014 Qual on same tyre compound
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MercedesCopyright: Mercedes
Mercedes: Back to base once again for Nico Rosberg - a little over 30mins remain and he's keen for more. He's a machine!!
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Nico Rosberg is back in the pits after that quick lap, leaving just Felipe Nasr and Valtteri Bottas out on track.
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F1 journalist Dan Knutson in Barcelona: "Sir Frank Williams is here. His team's most recent victory was the 2012 Spanish Grand Prix with Pastor Maldonado. Speaking with Valtteri Bottas and Felipe Massa here at the same circuit where that victory occurred, you can sense their quiet confidence that the team's 115th grand prix win is imminent."
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That lap from Nico Rosberg, set on the medium compound, is half a second off Romain Grosjean's best, with the Lotus driver clocking the benchmark time on the supersoft compound.
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Mercedes: Top garage banter from Nico Rosberg on seeing a certain team member (who shall remain nameless) in their travel gear...'Erm, why are you in your travel gear? We're not done yet!' That's the spirit you want from a driver!! ;)
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Vrooom! It is a quick lap from Nico Rosberg as a 1:24.584 sees him leap up to second.
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Nico Rosberg joins Sauber's Felipe Nasr in heading back out on to the circuit. Rosberg has 114 laps under his belt and clear wants more. He is out on mediums.
Daniil Kvyat, knocked off the top of the timesheet by Romain Grosjean, returns to the circuit for Red Bull as Nico Rosberg heads in. Just under one hour left.
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Nico Rosberg nips ahead of Nico Hulkenberg as a 1:26.414 is enough for him to claim the sixth fastest time. He's 2.3 seconds off Romain Grosjean's best effort.
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LotusCopyright: Lotus
Lotus: Only two purple sectors on that lap @RGrosjean ? #MustTryHarder ;-) P1 by almost 9 tenths currently
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Not long after going top, Romain Grosjean strengthens his position with a 1:24.067 on the super soft Pirellis.
That's the best time of the week and means Lotus are set to top three of the four days in Barcelona.
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WilliamsCopyright: Williams
Williams: Just under an hour and a quarter left of the session as Valtteri Bottas heads back out on track.
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Getty ImagesCopyright: Getty Images
F1 journalist Dan Knutson: "Valtteri Bottas had completed 95 laps today before his Williams FW37 stopped out on the circuit and had to be brought back on the truck."
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Blink and you will miss it. Nico Rosberg is on his 107th lap and this could be a big hitter with well over an hour left.
In contrast, McLaren as a team have managed just less than double that in the two tests so far.
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Honourable mention for Felipe Nasr and Sauber, who are up to 72 laps for the day. They'd manage just three by lunch after problems in the morning.
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Mercedes: Heads up! Nico Rosberg knocks it outta the park with a century of laps! And he's not done yet - 1hr 20mins left...
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It is pretty quiet out on the circuit with Daniil Kvyat, Felipe Nasr and Nico Rosberg cranking up the lap count. Rosberg is closing in on 100 for the day.
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LotusCopyright: Lotus
Lotus: Success for Romain Grosjean in the 'go fastest' simulation.
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No messing about from Romain Grosjean. He puts his foot on the pedal and slings the Lotus round the circuit in 1:24.681 to go fastest of all today.
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Just like before, it is Daniil Kvyat and Romain Grosjean champing at the bit to get back out and they do so immediately, with Nico Rosberg and Felipe Nasr joining them.
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Green flag and we are go again with 90 minutes left.
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Valtteri Bottas' Williams came to a halt at Turn Four, three minutes after the circuit had gone green.
Red flag
And another red flag, this time for Valtteri Bottas it would seem.
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Getty ImagesCopyright: Getty Images
Daniil Kvyat, Romain Grosjean and Valtteri Bottas are the first to return to the circuit with just over 90 minutes left of the final day of the second test.
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Carlos Sainz's Toro Rosso has been dealt with and the circuit is green again.
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McLaren confirm there will be no outing for Jenson Button this afternoon, but the main thing is that Fernando Alonso is uninjured after the earlier accident.
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McLaren: "That's the end of our programme here at the first Barcelona test. Thank you for all your messages of support for Fernando."
"Scans taken at the hospital show that Fernando Alonso has no injuries, a McLaren spokesperson says. It not yet decided if he will remain in the hospital overnight."
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F1 journalist Dan Knutson in Barcelona: "For most of the day the wind was blowing between 10 and 15 km/h, but now it has died down and is blowing between 0 and 5 km/h.
"The cars will be more stable on the track now, and things are more pleasant for the spectators in the afternoon sun."
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No more running from Force India today then.
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While we wait for more information on this stop, Force India say Nico Hulkenburg's car stopped earlier because of a power unit issue which they will "investigate back at the factory."
Red flag
Another red flag, with Carlos Sainz stopping at Turn Three in the Toro Rosso.
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Felipe Nasr, also on the soft compound, improves on his 26th lap of the day as a 1:25.487 moves him up to second, 0.546 off Daniil Kvyat's benchmark effort.
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Rapid from the Red Bull as Daniil Kvyat, on softs, flies around the circuit in 1:24.941 to go top of the timesheet. That's six tenths off the fastest time of the whole test, set by Pastor Maldonado on Saturday.
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F1 journalist Dan Knutson in Barcelona: "Valtteri Bottas' long run has been interrupted by planned visits to the Williams garage and the red flag for Nico Hulkenberg. He started on the soft compound Pirellis and switched to the medium slicks for just one lap before the scheduled stop in the garage. He then headed out on the hard compound slicks and had done 30 laps of the run before the red flag. He's out there again now and still on the hard tyres."
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Valtteri Bottas also heads back out in the Williams.
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Sebastian Vettel has been speaking about Fernando Alonso's accident. The Ferrari driver was right behind Alonso at the time of the incident, and said: "The speed was slow - maybe 150kph. Then he turned right into the wall. It looked strange."
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Green again and Carlos Sainz and Nico Rosberg get straight back on to the circuit.
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The Force India is on the back of the recovery truck and you would imagine we will be back under way shortly.
That's the third red flag of the day, and there is apparently smoke coming from the rear of Nico Hulkenberg's VJM07.
Red flag
Red flag, Nico Hulkenberg coming to a stop at Turn One.
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Force India: Nico completed his instal lap following some lunchtime checks and changes and is now heading out for his first run proper.
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Nico Hulkenberg does indeed depart the Force India garage, but only for a brief foray on to the circuit before returning to the pits.
Valtteri Bottas and Romain Grosjean, meanwhile, are lapping in the 1:30s as they continue their race sims.
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Force IndiaCopyright: Force India
Force India: The number 27 car is ready to depart the number 27 garage.
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BBCCopyright: BBC
F1 journalist Dan Knutson in Barcelona: "While they still put the screens up in front of the garage doors, the teams are not being as paranoid as they were about concealing their cars when the driver comes in and they push the car into the garage. By now most visible secrets the cars had at the start of testing are known to rival teams. Plus these are the interim-spec cars that will be obsolete after being updated at the next test with new race-spec parts that will be used in the Australian Grand Prix."
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Felipe Nasr is up to 16 laps for the day and his latest trip round the circuit sees him record a 1:26.121 on softs, which moves him up to second, just over half-a-second behind pacesetter Carlos Sainz.
Sebastian Vettel and Daniil Kvyat are back in the pits and the circuit is briefly empty, before Valtteri Bottas re-emerges with hard tyres bolted on his Williams.
McLaren statement
McLaren have released a statement regarding Fernando Alonso. Here it is in full:
"Today at 12:35 CET, while testing at the Circuit de Catalunya (Barcelona), during the fourth and final day of the current test, Fernando Alonso's McLaren-Honda car left the track at Turn Three, causing the right-hand side of his car to strike the wall.
"Fernando was driven to the circuit's Medical Centre where the circuit's doctors gave him first aid.
"He was conscious and spoke with the doctors.
"However, as per usual procedure in such circumstances, he was then airlifted to hospital where he is undergoing precautionary checks.
"We will issue a further update in due course."
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Sebastian Vettel kicks off this run with a 1:30.100 before Red Bull's Daniil Kvyat heads out and a personal best for today on his 50th lap, a 1:26.935, sees him climb into fifth.
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Carlos Sainz returns to the pits and switches off his Toro Rosso, meaning that is his running done for a little while. Meanwhile, Sebastian Vettel and Nico Rosberg are out on the circuit.
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That leaves three cars on the track, the Toro Rosso of Carlos Sainz, Felipe Nasr's Sauber and Romain Grosjean's Lotus.
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Valtteri Bottas is not out long, though, he heads straight back in.
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WilliamsCopyright: Williams
Like Romain Grosjean, Williams' schedule for this afternoon involves a race a simulation and Valtteri Bottas pits after completing 16 laps in this programme. He takes on mediums and heads back out.
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That time is not enough to budge Felipe Nasr off the bottom of the timesheet, but his next one is as he goes round in 1:27.366 to leap up to fifth.
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After a morning of relative inactivity, Felipe Nasr gets off the mark with a 1:31.906 on the soft tyre.
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Romain Grosjean pits for mediums and heads back out. Sebastian Vettel, on the hard tyre, is lapping in the 1:30s.
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Sebastian Vettel is out on track now, joining Nico Rosberg, Daniil Kvyat, Valtteri Bottas and Romain Grosjean, whose latest lap time is 1:33.
Felipe Nasr has been out in the Sauber, but is back in now.
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SauberCopyright: Sauber
Sauber's Felipe Nasr only managed three laps before "general issues" with the C34 meant he spent most of his time in the team's garage.
It looks like those issues have been fixed, because Sauber tweets saying he is about to head back out.
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Romain Grosjean is on the soft tyres and lapping in the 1:30s while Nico Rosberg, on mediums, is lapping around a second faster.
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Lotus Copyright: Lotus
Lotus: Come on Romain Grosjean, it must be time for an afternoon race simulation? Let's see what you've got.
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Stuie Neale: Good pace from Carlos Sainz, Vettel maybe proving a point to Red Bull with good pace
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Romain Grosjean also heads out, as does current pacesetter Carlos Sainz and then Nico Rosberg.
Testing resumes
WilliamsCopyright: Williams
The track goes green and Valtteri Bottas is straight out in the Williams.
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McLaren: Fernando is conscious and is speaking. He has been airlifted to hospital for further checks.
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There is a slight delay to the second half of testing, which was due to start at 13:00 GMT. It will now start at 13:15.
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BBCCopyright: BBC
F1 journalist Dan Knutson in Barcelona: "Romain Grosjean chatted with the media during the lunch break. He says the Lotus E23 responds well to driver input, but the windy conditions make it things tricky out on the track. The plan for this afternoon is long runs and possibly a race simulation."
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Heikki Kovalainen: All the best to Fernando Alonso & McLaren, waiting for more news, hope all is well!
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ESPNF1Copyright: ESPNF1
ESPN F1: Here's where Alonso left the circuit at Turn 3 exit - a long way back from where he stopped.
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Tom Watts: Slightly concerning about Alonso - not much damage on the car, so is there a reason other than injury for being taken to hospital?
Everything would suggest it was a precautionary measure, Tom.
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Jennie Gow
BBC Radio 5 live pit-lane reporter
"FIA say Alonso is conscious. They report he spoke to doctors before being flown to hospital for checks. We await more from McLaren."
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Jennie Gow
BBC Radio 5 live pit-lane reporter
"Alonso's manager tweets: 'Fernando is OK and conscious. Thank you !' No more news."
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Getty ImagesCopyright: Getty Images
A large crowd had gathered close to the helicopter which airlifted Fernando Alonso to hospital, where he is confirmed as being conscious.
Another picture just before Fernando Alonso was airlifted to hospital.
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McLaren confirm that Fernando Alonso has been airlifted to hospital and that he is conscious.
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Getty ImagesCopyright: Getty Images
This is the scene of Fernando Alonso's crash.
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A further update from the Circuit De Catalunya twitter feed: "It seems that Fernando Alonso is ok. We hope that everything's alright and all ends with just a shock!!"
No official confirmation from McLaren as yet, but will keep you updated.
Final day testing summary
Here is a summary of the key moments from this morning:
Alonso crashes just before end of morning session
McLaren did not emerge until two hours into testing
Carlos Sainz fastest with 1:25.605
Rosberg spins early on but goes on to complete 67 laps
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The Circuit De Catalunya tweets a further update: "Fernando Alonso crashed in the end of Turn 3 (the 'curvone'). The car has gone back and crashed into the wall."
Still waiting for news regarding that stop for Fernando Alonso, who is unlikely to see any more action today with 15 minutes until lunch and Jenson Button stepping into the McLaren in the afternoon.
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It is indeed the McLaren, which has come to a stop at Turn Four.
Red flag
Red flag, and it looks like it could be Fernando Alonso's McLaren that has stopped...
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Another improvement from Fernando Alonso who, on medium tyres, completes his 20th lap in 1:27.956. That's the fifth fastest time of the day.
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Fernando Alonso is up to 16 laps in the 90 minutes or so he has been able to get time out on the circuit, with his best time a 1:28.308 - seventh fastest.
He briefly drops to eighth when Daniil Kvyat moves up to sixth with a 1:28.100 but then clocks 1:28.308 to move 0.279s behind Romain Grosjean, who has the fifth fastest time.
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Force IndiaCopyright: Force India
Nico Hulkenberg heads out in the Force India and is round in 1:26.591 on softs, moving up to third before returning to the pits.
Carlos Sainz, meanwhile, is making marginal gains, improving his timesheet-topping time to 1:25.604, 0.08s faster than his previous best.
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Daniil Kvyat is heading back out in the Red Bull, with Carlos Sainz and Nico Rosberg going out again too with just under 40 minutes until lunch.
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Getty ImagesCopyright: Getty Images
F1 journalist Dan Knutson in Barcelona: "This has been a common sight during the eight days of pre-season testing - an empty Mercedes garage while either Nico Rosberg or Lewis Hamilton is out on track racking up the laps in the reliable W06."
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Fernando Alonso and Romain Grosjean make a brief appearance on the circuit, both on mediums, before heading back into the pits. Sebastian Vettel also heads in as his times on the softs begin to tail off, leaving Valtteri Bottas alone on the circuit.
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MercedesCopyright: Mercedes
Nico Rosberg brings up his half century of laps as Sebastian Vettel and Carlos Sainz rejoin him on the circuit.
Meanwhile, Daniil Kvyat is improving, his latest effort in the Red Bull a 1:29.982.
"It has been a low-key winter for Williams so far, with few attention-grabbing lap times, even if Felipe Massa was third fastest on Friday at this test. But there is a quiet confidence that they will have a strong season.
"Valtteri Bottas, who continues in the car this morning after taking over from Massa at lunchtime on Saturday, said: 'I think it's going to be close this year. It is clear Ferrari have improved and possibly Red Bull. We have, and Mercedes is going to be strong again. But I think it is going to be close. We will be there or thereabouts.'"
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WilliamsCopyright: Williams
Williams: A few changes and Valtteri Bottas heads back out. So far he has done 36 laps.
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Nico Rosberg is back on the circuit, with Sebastian Vettel and Carlos Sainz returning to the pits after that short stint. Rosberg, on medium tyres, improves with a 1:26.782 but remains third fastest.
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Force India: As we hit noon in Spain, Nico has completed 26 laps with a best of 1:27.561. The Hulk is back in the car, ready for his next run.
Chris Akrill: There's plenty of people here, we're all in the sun!
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Carlos Sainz is also out on the circuit and, like Sebastian Vettel, is running on soft tyres. He knocks Vettel off top spot with a 1:25.684.
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Sebastian Vettel is out on softs and he clocks the fastest time of the day so far, a 1:26.454.
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So far so good for the patched-up McLaren and Fernando Alonso, with the new fix applied to their faulty seal seemingly doing the business this morning. Alonso is not on the circuit at the moment, with Sebastian Vettel's Ferrari providing the only on-circuit activity.
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Quiet again in Barcelona with everyone back in the pits. Mercedes continue to tick over the mileage clock with Nico Rosberg having got 46 laps under his belt this morning.
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Matthew Mc20nnell: Will be great to see McLaren's pace in Australia once they've turned the engine up.
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Fernando Alonso is back out and putting some good laps in. He improves with a 1:28.308 as he takes his lap count into double figures.
Mark Jones: McLaren car and Honda engine will be strong once reliability sorted. I can see McLaren getting podiums this year.
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McLarenCopyright: McLaren
McLaren: And we're away. Fernando takes the MP4-30 for a spin in the Barca sun.
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A tenth of a second separates the top three as Nico Rosberg goes fastest with a 1:27.142, putting him just ahead of Carlos Sainz.
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BBCCopyright: BBC
F1 journalist Dan Knutson in Barcelona: "There were more fans in the grandstand but they drifted away after getting bored waiting for Fernando Alonso to appear in the McLaren-Honda. The fact that Alonso will only drive for half the day, and that the car has completed so few laps over the past three days no doubt kept other fans at home."
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Force India: The main focus of today's work is to allow Nico to have an in-depth assessment of the new 2015 tyres, together with mileage.
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And now Daniil Kvyat puts a time on the board, a 1:42.118 on his 24th lap before improving to 1:34.031.
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Fernando Alonso is making up for lost time as he completes his sixth lap of the circuit. As it stands, Felipe Nasr and Daniil Kvyat are the only drivers still to record timed laps. Nasr is back in the Sauber garage as the team looks to resolve an issue that has hampered their running today, while Kvyat has been working through a programme of untimed runs, like he did yesterday morning.
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John Gilmore: If McLaren turn up the power at end of test other teams will head to first race not knowing what the car is capable of - #mindgames.
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There it is. Fernando Alonso, on medium tyres, gets off the mark with a 1:30.427, the seventh fastest of the morning so far, but immediately improves with 1:28.364.
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Fernando Alonso is back out on the circuit and we could be about to see a first timed effort of the day from the McLaren.
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Sebastian Vettel ends a brief moment of circuit silence, with everyone else back in the pits.
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Toro RossoCopyright: Toro Rosso
Toro Rosso: Sun is shining also from the Fastest Laps screen... Carlos Sainz is on top!
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Finally, the sight McLaren fans, and the Spanish spectators in the stands, have been waiting to see - Fernando Alonso heads out in the McLaren and completes his first lap of the day.
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Scott Ambrose: No surprise at McLaren's initial problems but do think that car will have plenty of pace and power - eventually.
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McLarenCopyright: McLaren
Former Barcelona president Joan Laporta has paid McLaren a visit this morning.
I don't know about you, but Fernando Alonso has the look of a man desperate to be anywhere else but there. Like, the circuit for example.
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Not much to separate Sebastian Vettel, Nico Rosberg and Carlos Sainz at the top of the order, with Sainz reclaiming top spot as he clocks 1:27.236 on softs.
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F1 journalist Dan Knutson in Barcelona: "Williams planned to do pit stop practice today and chassis set-up work yesterday, but with the patchy rain on Saturday the team switched things around so that the drivers could lap on a dry track today. Felipe Massa and Valtteri Bottas each did 20 pit stops yesterday, so the crew got a real workout."
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Romain Grosjean has been making marginal improvements with each lap and is currently fourth fastest with a 1:28.029. Nico Rosberg is also improving as a 1:27.663 puts him just 0.013s off Sebastian Vettel. Both are on the medium tyre.
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A reminder that the plan is for Fernando Alonso and Jenson Button is to share track time today, so neither will probably want these delays to go on much longer.
"A McLaren spokeswoman says: 'The new part (MGU-K) arrived late last night so we needed time for the power unit reconfiguration. We should be on track very very soon.'"
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Romain GrosjeanCopyright: Romain Grosjean
Lotus: 17 laps on the board, latest tweaks completed in the garage, Romain Grosjean returns trackwards.
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Nico Rosberg dips below 1:30 for the first time today with a 1:28.216, before shaving off 0.041s to close in on Sebastian Vettel's best effort.
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Nico Hulkenberg heads back out, as does Sebastian Vettel and the latter, on medium tyres, storms to the top of the timesheet with a 1:27.474.
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Getty ImagesCopyright: Getty Images
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Chris Akrill: McLaren in for a testing season, playing catch-up to the likes of Ferrari. If they spring a surprise will run barefoot to Marenello!
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WilliamsCopyright: Williams
Williams: Valtteri is wheeled back into the garage after his latest run. It's windy out on track which will mix things up a bit.
"There is an update from McLaren. They say they will be out on track in 40 minutes and will come back to us with more details on the reason for the delay."
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Silence on the circuit, with everyone back in the pits. One hour and 30 minutes of testing gone and still no McLaren.
"Sauber caught the eye at the first test in Jerez with a fastest time of all on one day, and second fastest on the other three. But Marcus Ericsson, one half of an all-new driver line-up for the Swiss team this year, admitted on Saturday that this was solely down to the team running the car in a different spec from the others. 'We are not fooling ourselves and thinking we will fight for P1,' the Swede said. 'We are aiming to fight for points. We are happy with the pre-season so far and hopefully we can continue that trend.'
"Sauber had the worst season in their history last year, failing even to score a point. Ericsson said the team had made 'for sure a step, on the car and the power unit, although how big is difficult say. It's promising but we have to wait and see.'"
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Sauber's Felipe Nasr managed just three laps this morning and he is currently back in the team garage.
They confirm they are having "general issues with the C34", adding that they are investigating the problem and aim to be back on the circuit as soon as possible.
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Nico Rosberg is clearly unperturbed by his early venture off the circuit, he has passed the 20-lap mark, meaning he has more mileage under his belt than any other driver so far today.
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Feeling Ire: Can you imagine if this year's McLaren turns out to be like the MP4-18 of 2003 and never races! I wonder what Alonso's thinking.....
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As is often the case, the moment I rattle off an update on times someone only goes and sets a new fastest lap.
Carlos Sainz, on softs, clocks 1:27.991 to take Toro Rosso to the top of the timesheet.
An improvement from Nico Hulkenberg as he completes a lap of the circuit in 1:30.221. He has the third fastest time of the day, behind Sebastian Vettel and pacesetter Valtteri Bottas.
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Gary Brown: Never underestimate HONDA the car is yet to run at full power and the chassis seems good might be a surprise in Melbourne.
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Force IndiaCopyright: Force India
Force India: Car 27 leaves the pits as we return to track. Nico out to set his first times laps of the day.
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Nico Hulkenberg joins Romain Grosjean, Carlos Sainz, Nico Rosberg, Sebastian Vettel and Valtteri Bottas in getting a time on the board, a 1:31.251.
"I don't want to be a doom monger or anything, but it's now an hour into the final day of this test and the McLaren-Honda has still not gone out. Honda said earlier that it was 'time to give the power unit a final run with more testing. Stability is the main focus.' In which case, you have to ask, what's keeping them, if it's not yet another problem?"
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Speaking of McLaren, there's still no sign of them on the circuit with one hour of testing gone...
"Unlike McLaren, this has been a good test for Lotus, of whom Pastor Maldonado has topped the times on two of the three days so far. The team had a terrible season in 2014, with an unpredictable car lacking downforce and down on power because of the Renault engine. Having switched to Mercedes, and with a completely new car, Maldonado says he is encouraged. 'The car seems to be very consistent,' the Venezuelan said. 'I don't know if it is fast. There is a lot to do. We are not at the top of the potential of the car but it is quite positive."
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Valtteri Bottas storms to the top of the embryonic timesheet with his first flyer, going round in 1:28.868.
Pastor Maldonado has the benchmark time, setting 1:24.348 in the Lotus on Saturday.
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Nico Rosberg's Mercedes has had the last remnants of gravel hoovered out of it and is back on the circuit. He clocks 1:33.627 to go second.
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Matthew Traynor: McLaren-Honda will surprise everyone with how competitive their car will be. Note, Red Bull did less miles last winter.
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Force IndiaCopyright: Force India
Force India: Fearless fans braving the freezing cold this morning: we salute you!
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The Ferrari driver was fifth fastest on Saturday, the lowest he or team-mate Kimi Raikkonen have finished in a test session all winter, but the former world champion was unconcerned.
"We have to go day by day, it's difficult to say because (Saturday) was quite cold," said Vettel.
"Maybe not as good as the days before but then again it's the same for everyone. The most important thing is the car continues to run and without any big problems."
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Sebastian Vettel, who racked up 105 laps on Saturday, keeps shaving a bit off his time with each lap, improving to 1:31.562, then 1:31.332 and now 1:30.363.
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Sebastian Vettel improves on Nico Rosberg's 1:36 by more than four seconds, clocking 1:31.990 with his first timed lap.
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Mercedes: Cold track, cold tyres, a simple spin for Nico Rosberg. A few high tech (ahem) gravel hoovers required but no major dramas to report.
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Mark Jones: I don't think McLaren should be overly concerned. Integration on new Honda was always going to be tough. Look where Renault were!
"Sebastian Vettel has suggested he might defy the newly imposed ban on drivers changing their helmet designs. The German, whose penchant for using a new design at virtually every race has partially led to the ban, said: 'The plan was to calm down a bit. But it depends on the penalty, if it is a little fine with some money for charity, I am happy to keep changing my helmet.'"
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Romain Grosjean also heads on to the circuit, before Sebastian Vettel takes the Ferrari out to join them. Vettel, by the way, has been in a rebellious mood this week...
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SauberCopyright: Sauber
Nico Rosberg is back at the Mercedes garage and we are green again. Felipe Nasr is straight back out on to the circuit.
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MercedesCopyright: Mercedes
Mercedes: Plenty of attention (a little too much from this particular cameraman!!) on Nico Rosberg as he returns to base...
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F1 journalist Dan Knutson in Barcelona: "We've seen more drivers go off here than at Jerez. Sebastian Vettel, who spun off on his first lap yesterday, said it was a lot colder and therefore more slippery here than at Jerez."
Get involved
As we wait for Nico Rosberg's Mercedes to be recovered, now seems as good a time as any to get your thoughts on all things Formula 1, and check that you are all awake.
Are McLaren's testing woes a cause for concern? Who do you think will be the biggest surprise this season?
Let us know via #bbcf1 and text in on 81111 (UK only)
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Mercedes: Hmmm... monitors suggest Nico Rosberg has come to a halt at T5... best get walking hadn't we?
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Williams: Looks like Nico Rosberg has found the snow (thanks @neillmcreynolds) #redflag
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Nico Rosberg's off was at Turn 5, the recovery truck driver has had his morning coffee cut short and he is in the vehicle and on his way to the stricken Mercedes.
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It is for Nico Rosberg's Mercedes, which is in the gravel.
Red flag
And out pops the red flag for the first time today.
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Just over 20 minutes in and Nico Rosberg has completed six laps, with Carlos Sainz and Daniil Kvyat having been round the circuit four times already.
"Lewis Hamilton's work is done for this week and he can leave Barcelona and concentrate on getting well again after feeling under the weather all weekend. A fever forced him to stop testing after a few laps on Thursday, and he has not been 100% for the last two days, despite completing the best part of 200 laps in a day and a half. Hamilton was unhappy with the new Pirelli tyres on Saturday, complaining that he hardly ever got them all up to temperature at the right time, and that having to focus on doing so was interfering with testing. But he said the car 'feels fine'. Things again looked good for him and Mercedes on Saturday - he was comfortably the quickest on the medium tyre despite not doing any performance runs. He remains a justified favourite for a third title."
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Nico Rosberg is the first to put a time on the board with a 1:36.999. The Mercedes driver is back in the car today after Lewis Hamilton completed his Barcelona testing on Saturday.
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Install laps done, a quick return to the pits for a chat with the engineers and the cars are heading back out.
"Button continued: 'We are not going have a race-winning car at the first race but we might at the last race. You are going to see big improvements from us. We are all quite interested to see what this car can do when we are at full power and have done some set-up work because it has a lot of potential but it is very difficult extract right now.'"
"Jenson Button has - so far - had the worst of the unreliability at McLaren, although Fernando Alonso is not exactly brimming with mileage either. It seems a proper fix for the hybrid system seal problem that hit the team again on Saturday is still some way away and both drivers are likely to spend a fair bit of time in the pits on Sunday. 'I don't think we'll be doing 150 laps,' Button said. But there is a confidence that once the problems with the engine can be solved, this is a good car, unlike those of 2013 and '14.
"'We're not in for another tough season,' Button said. That's definitely not the case but whether we are ready for the first race is another question. The last two years we just weren't quick enough, whereas now everyone else out there has so much experience with these power units and Honda doesn't so it looks worse than it is. There are problems we need to solve, and we will, and hope the next test is good in terms of mileage so we can feel confident going into the first race."
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The "beautiful day" I mentioned in my opening entry refers to the comment made by Fernando Alonso after Friday's testing, when the Spaniard completed 59 laps - the most McLaren have managed in a single day of testing so far.
"It has been a beautiful day, driving the car and exploring some of the potential," he said.
"Obviously we are running with some limitations still on the engine and full power etc, but definitely the car felt better than Jerez."
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Honda RacingCopyright: Honda Racing
Honda Racing F1: Last day of test 2, time to give the power unit a final run with more testing. Stability is the main focus for today.
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No messing about from today's drivers as Nico Hulkenberg heads out for the first time in testing, with Daniil Kvyat, Nico Rosberg and Felipe Nasr in close company. Just waiting on McLaren.
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F1 journalist Dan Knutson in Barcelona: "A nice, sunny Sunday. The air temperature is 7.3 degrees Centigrade, and the track temperature is 11.1 degrees Centigrade."
Today's line-up
Here's who is scheduled to run today:
Mercedes: Nico Rosberg
Red Bull: Daniil Kvyat
Williams: Felipe Massa and Valtteri Bottas
Ferrari: Sebastian Vettel
McLaren: Fernando Alonso and Jenson Button
Lotus: Romain Grosjean
Toro Rosso: Carlos Sainz
Sauber: Felipe Nasr
Force India: Nico Hulkenberg
Go! Go! Go!
The circuit is open and out go Valtteri Bottas, Romain Grosjean, Carlos Sainz and Sebastian Vettel, who might be a little more tentative on his early foray on to the track after spinning yesterday.
"McLaren have changed their plans for the final day of this test following their reliability problems on Saturday. The schedule had been for Fernando Alonso to do all the driving on Sunday, but the lack of mileage achieved by Jenson Button over Thursday and Saturday - a total of 33 laps over two days, compared with the 59 Alonso did on Friday - has forced a rethink. A spokeswoman said: 'We wanted to have balanced feedback from the drivers, thus we needed to give them the chance of having equal mileage.'"
Button back in the seat
McLarenCopyright: McLaren
Jenson Button may have spent more time sitting in the McLaren garage than he has in the car in Barcelona, but he gets another chance today as the team confirms the Briton and Fernando Alonso will be sharing testing duties. Alonso is out in the morning, with Button taking over in the afternoon.
"Not for the first time this winter, McLaren start a day's testing in desperate need of a good run. And the longer their troubles continue, the more desperate that need gets. Saturday was another terrible disappointment from the team, as a newly redesigned part fitted to the Honda engine to fix a previous failure itself broke within just a few laps. The part - a seal in the hybrid system - has been re-designed for a third time and Honda and McLaren will have everything crossed that this time it has worked, and they can end this penultimate test with a decent day's running."
Time for another beautiful day?
Just 24 laps completed...
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...and the slowest driver on show...
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...it has been a frustrating, and potentially worrying, couple of days of testing for Jenson Button and McLaren in Barcelona.
Live Reporting
Gary Rose
All times stated are UK
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Getty ImagesCopyright: Getty Images - Alonso crashes just before end of morning session
- McLaren did not emerge until two hours into testing
- Carlos Sainz fastest with 1:25.605
- Rosberg spins early on but goes on to complete 67 laps
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Latest PostPost update
OK then, time for me to sign off after an intriguing couple of days. Only one test remains, in Barcelona, and we will be back 08:00 GMT sharp on Thursday for live text coverage of all four days.
I'll be back in three weeks for the season opener in Australia.
Bring it on.
Get involved #bbcf1
Nick Bennetts: Only 3 weeks until Melbourne. Hurry up F1!
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Mercedes: 131 laps for @nico_rosberg to cap off our week @Circuitcat_eng - top job everyone!
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So it is Romain Grosjean who finishes with the fastest time of the week, but that was set on supersoft tyres, meaning Nico Rosberg's 1:24.321 on medium tyres today was arguably the lap of the week, capping another impressive day for Mercedes.
Testing day four
1. Romain Grosjean (Lotus) 1:24.067 (111 laps)
2. Nico Rosberg (Mercedes) 1:24.321 (130 laps)
3. Daniil Kvyat (Red Bull) 1:24.941 (103 laps)
4. Felipe Nasr (Sauber) 1:24.956 (73 laps)
5. Valtteri Bottas (Williams) 1:25.345 (128 laps)
6. Carlos Sainz (Toro Rosso) 1:25.605 (88 laps)
7. Sebastian Vettel (Ferrari) 1:26.312 (74 laps)
8. Nico Hulkenberg (Force India) 1:26.591 (36 laps)
9. Fernando Alonso (McLaren) 1:27.956 (20 laps)
Chequered flag
The chequered flag arrives as Valtteri Bottas and three others head out. The second pre-season test is over.
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Williams: Green light so Valtteri heads out!
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Green flag with a solitary minute left. Any takers?
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Andrew Benson
Chief F1 writer
"McLaren team boss Eric Boullier says Fernando Alonso will stay in hospital overnight."
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Mercedes: 3mins to go... will we, won't we?
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Felipe Nasr had certainly been making up for lost time, having completed 70 laps this afternoon before that stop.
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Toro Rosso: "Early finish for Carlos: a small mistake. But still a long, solid job: Laps 88, Best 1:25.604."
Red flag
Into the final few minutes and we have another red flag, with the Sauber of Felipe Nasr at a standstill at Turn Six.
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F1 journalist Dan Knutson in Barcelona: "After two hours and 45 minutes in the garage, Sebastian Vettel is back on track. He has the hard compound Pirellis on his Ferrari."
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Lotus: 111 laps completed, fastest time of the day (so far), we'll call that a day.
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Romain Grosjean has the fastest lap of the day, will he look to improve on that? No, Lotus confirms his testing for today is done.
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Daniel Ricciardo: Glad to hear Fernando Alonso all ok.
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We've not seen this chap for a while - Sebastian Vettel. He finally re-emerges in the Ferrrari with 15 minutes.
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Nico Rosberg is back out on medium tyres and he shaves a couple of tenths off his previous best, clocking 1:24.321 to leave him just two tenths slower than Romain Grosjean's leading time.
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F1 journalist Dan Knutson in Barcelona: "The transporters will be staying put in the paddock as there are only three days between now and the next test here at the Circuit de Catalunya.
"The season starts with four flyaway races - Australia, Malaysia, China and Bahrain - so the next time the transporters will appear will be here in May at the Spanish Grand Prix which is the first race of the European season."
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Nico Rosberg is back out for the final 25 minutes of the second pre-season test. To highlight just how impressive that lap by Rosberg was, there is a 1.5-2secs difference between the medium and the supersoft.
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Williams: 122 laps so far as Valtteri Bottas heads off down the pitlane
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Romain Grosjean is back out in the Lotus but not for long as he makes a swift return to the pits. Valtteri Bottas heads back out as he looks to add to his impressive lap tally for today.
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WelshRacer: Rosberg time of 1m24.584 on medium compound is 9 tenths quicker than he did in 2014 Qual on same tyre compound
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Mercedes: Back to base once again for Nico Rosberg - a little over 30mins remain and he's keen for more. He's a machine!!
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Nico Rosberg is back in the pits after that quick lap, leaving just Felipe Nasr and Valtteri Bottas out on track.
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F1 journalist Dan Knutson in Barcelona: "Sir Frank Williams is here. His team's most recent victory was the 2012 Spanish Grand Prix with Pastor Maldonado. Speaking with Valtteri Bottas and Felipe Massa here at the same circuit where that victory occurred, you can sense their quiet confidence that the team's 115th grand prix win is imminent."
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That lap from Nico Rosberg, set on the medium compound, is half a second off Romain Grosjean's best, with the Lotus driver clocking the benchmark time on the supersoft compound.
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Mercedes: Top garage banter from Nico Rosberg on seeing a certain team member (who shall remain nameless) in their travel gear...'Erm, why are you in your travel gear? We're not done yet!' That's the spirit you want from a driver!! ;)
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Vrooom! It is a quick lap from Nico Rosberg as a 1:24.584 sees him leap up to second.
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Nico Rosberg joins Sauber's Felipe Nasr in heading back out on to the circuit. Rosberg has 114 laps under his belt and clear wants more. He is out on mediums.
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Sauber: Back on track with the supersoft tyres.
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Daniil Kvyat, knocked off the top of the timesheet by Romain Grosjean, returns to the circuit for Red Bull as Nico Rosberg heads in. Just under one hour left.
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Nico Rosberg nips ahead of Nico Hulkenberg as a 1:26.414 is enough for him to claim the sixth fastest time. He's 2.3 seconds off Romain Grosjean's best effort.
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Lotus: Only two purple sectors on that lap @RGrosjean ? #MustTryHarder ;-) P1 by almost 9 tenths currently
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Not long after going top, Romain Grosjean strengthens his position with a 1:24.067 on the super soft Pirellis.
That's the best time of the week and means Lotus are set to top three of the four days in Barcelona.
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Williams: Just under an hour and a quarter left of the session as Valtteri Bottas heads back out on track.
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F1 journalist Dan Knutson: "Valtteri Bottas had completed 95 laps today before his Williams FW37 stopped out on the circuit and had to be brought back on the truck."
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Blink and you will miss it. Nico Rosberg is on his 107th lap and this could be a big hitter with well over an hour left.
In contrast, McLaren as a team have managed just less than double that in the two tests so far.
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Honourable mention for Felipe Nasr and Sauber, who are up to 72 laps for the day. They'd manage just three by lunch after problems in the morning.
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Mercedes: Heads up! Nico Rosberg knocks it outta the park with a century of laps! And he's not done yet - 1hr 20mins left...
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It is pretty quiet out on the circuit with Daniil Kvyat, Felipe Nasr and Nico Rosberg cranking up the lap count. Rosberg is closing in on 100 for the day.
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Lotus: Success for Romain Grosjean in the 'go fastest' simulation.
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No messing about from Romain Grosjean. He puts his foot on the pedal and slings the Lotus round the circuit in 1:24.681 to go fastest of all today.
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Just like before, it is Daniil Kvyat and Romain Grosjean champing at the bit to get back out and they do so immediately, with Nico Rosberg and Felipe Nasr joining them.
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Green flag and we are go again with 90 minutes left.
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Valtteri Bottas' Williams came to a halt at Turn Four, three minutes after the circuit had gone green.
Red flag
And another red flag, this time for Valtteri Bottas it would seem.
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Daniil Kvyat, Romain Grosjean and Valtteri Bottas are the first to return to the circuit with just over 90 minutes left of the final day of the second test.
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Carlos Sainz's Toro Rosso has been dealt with and the circuit is green again.
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McLaren confirm there will be no outing for Jenson Button this afternoon, but the main thing is that Fernando Alonso is uninjured after the earlier accident.
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McLaren: "That's the end of our programme here at the first Barcelona test. Thank you for all your messages of support for Fernando."
Alonso has "no injuries"
Andrew Benson
Chief F1 writer
"Scans taken at the hospital show that Fernando Alonso has no injuries, a McLaren spokesperson says. It not yet decided if he will remain in the hospital overnight."
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F1 journalist Dan Knutson in Barcelona: "For most of the day the wind was blowing between 10 and 15 km/h, but now it has died down and is blowing between 0 and 5 km/h.
"The cars will be more stable on the track now, and things are more pleasant for the spectators in the afternoon sun."
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No more running from Force India today then.
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While we wait for more information on this stop, Force India say Nico Hulkenburg's car stopped earlier because of a power unit issue which they will "investigate back at the factory."
Red flag
Another red flag, with Carlos Sainz stopping at Turn Three in the Toro Rosso.
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Felipe Nasr, also on the soft compound, improves on his 26th lap of the day as a 1:25.487 moves him up to second, 0.546 off Daniil Kvyat's benchmark effort.
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Rapid from the Red Bull as Daniil Kvyat, on softs, flies around the circuit in 1:24.941 to go top of the timesheet. That's six tenths off the fastest time of the whole test, set by Pastor Maldonado on Saturday.
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F1 journalist Dan Knutson in Barcelona: "Valtteri Bottas' long run has been interrupted by planned visits to the Williams garage and the red flag for Nico Hulkenberg. He started on the soft compound Pirellis and switched to the medium slicks for just one lap before the scheduled stop in the garage. He then headed out on the hard compound slicks and had done 30 laps of the run before the red flag. He's out there again now and still on the hard tyres."
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Valtteri Bottas also heads back out in the Williams.
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Sebastian Vettel has been speaking about Fernando Alonso's accident. The Ferrari driver was right behind Alonso at the time of the incident, and said: "The speed was slow - maybe 150kph. Then he turned right into the wall. It looked strange."
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Green again and Carlos Sainz and Nico Rosberg get straight back on to the circuit.
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The Force India is on the back of the recovery truck and you would imagine we will be back under way shortly.
Day four testing update (as of 14:39 GMT)
1. Carlos Sainz (Toro Rosso) 1:25.605 (76 laps)
2. Felipe Nasr (Sauber) 1:26.121 (24 laps)
3. Daniil Kvyat (Red Bull) 1:26.179 (58 laps)
4. Sebastian Vettel (Ferrari) 1:26.312 (69 laps)
5. Nico Hulkenberg (Force India) 1:26.591 (36 laps)
6. Nico Rosberg (Mercedes) 1:26.782 (83 laps)
7. Fernando Alonso (McLaren) 1:27.956 (20 laps)
8. Romain Grosjean (Lotus) 1:28.029 (84 laps)
9. Valtteri Bottas (Williams) 1:28.171 (83 laps)
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That's the third red flag of the day, and there is apparently smoke coming from the rear of Nico Hulkenberg's VJM07.
Red flag
Red flag, Nico Hulkenberg coming to a stop at Turn One.
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Force India: Nico completed his instal lap following some lunchtime checks and changes and is now heading out for his first run proper.
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Nico Hulkenberg does indeed depart the Force India garage, but only for a brief foray on to the circuit before returning to the pits.
Valtteri Bottas and Romain Grosjean, meanwhile, are lapping in the 1:30s as they continue their race sims.
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Force India: The number 27 car is ready to depart the number 27 garage.
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F1 journalist Dan Knutson in Barcelona: "While they still put the screens up in front of the garage doors, the teams are not being as paranoid as they were about concealing their cars when the driver comes in and they push the car into the garage. By now most visible secrets the cars had at the start of testing are known to rival teams. Plus these are the interim-spec cars that will be obsolete after being updated at the next test with new race-spec parts that will be used in the Australian Grand Prix."
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Felipe Nasr is up to 16 laps for the day and his latest trip round the circuit sees him record a 1:26.121 on softs, which moves him up to second, just over half-a-second behind pacesetter Carlos Sainz.
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Lotus: And back to the track for Romain Grosjean.
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Sebastian Vettel and Daniil Kvyat are back in the pits and the circuit is briefly empty, before Valtteri Bottas re-emerges with hard tyres bolted on his Williams.
McLaren statement
McLaren have released a statement regarding Fernando Alonso. Here it is in full:
"Today at 12:35 CET, while testing at the Circuit de Catalunya (Barcelona), during the fourth and final day of the current test, Fernando Alonso's McLaren-Honda car left the track at Turn Three, causing the right-hand side of his car to strike the wall.
"Fernando was driven to the circuit's Medical Centre where the circuit's doctors gave him first aid.
"He was conscious and spoke with the doctors.
"However, as per usual procedure in such circumstances, he was then airlifted to hospital where he is undergoing precautionary checks.
"We will issue a further update in due course."
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Sebastian Vettel kicks off this run with a 1:30.100 before Red Bull's Daniil Kvyat heads out and a personal best for today on his 50th lap, a 1:26.935, sees him climb into fifth.
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Carlos Sainz returns to the pits and switches off his Toro Rosso, meaning that is his running done for a little while. Meanwhile, Sebastian Vettel and Nico Rosberg are out on the circuit.
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That leaves three cars on the track, the Toro Rosso of Carlos Sainz, Felipe Nasr's Sauber and Romain Grosjean's Lotus.
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Valtteri Bottas is not out long, though, he heads straight back in.
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Like Romain Grosjean, Williams' schedule for this afternoon involves a race a simulation and Valtteri Bottas pits after completing 16 laps in this programme. He takes on mediums and heads back out.
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That time is not enough to budge Felipe Nasr off the bottom of the timesheet, but his next one is as he goes round in 1:27.366 to leap up to fifth.
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After a morning of relative inactivity, Felipe Nasr gets off the mark with a 1:31.906 on the soft tyre.
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Romain Grosjean pits for mediums and heads back out. Sebastian Vettel, on the hard tyre, is lapping in the 1:30s.
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Sebastian Vettel is out on track now, joining Nico Rosberg, Daniil Kvyat, Valtteri Bottas and Romain Grosjean, whose latest lap time is 1:33.
Felipe Nasr has been out in the Sauber, but is back in now.
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Sauber's Felipe Nasr only managed three laps before "general issues" with the C34 meant he spent most of his time in the team's garage.
It looks like those issues have been fixed, because Sauber tweets saying he is about to head back out.
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Romain Grosjean is on the soft tyres and lapping in the 1:30s while Nico Rosberg, on mediums, is lapping around a second faster.
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Lotus: Come on Romain Grosjean, it must be time for an afternoon race simulation? Let's see what you've got.
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Stuie Neale: Good pace from Carlos Sainz, Vettel maybe proving a point to Red Bull with good pace
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Romain Grosjean also heads out, as does current pacesetter Carlos Sainz and then Nico Rosberg.
Testing resumes
The track goes green and Valtteri Bottas is straight out in the Williams.
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McLaren: Fernando is conscious and is speaking. He has been airlifted to hospital for further checks.
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There is a slight delay to the second half of testing, which was due to start at 13:00 GMT. It will now start at 13:15.
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F1 journalist Dan Knutson in Barcelona: "Romain Grosjean chatted with the media during the lunch break. He says the Lotus E23 responds well to driver input, but the windy conditions make it things tricky out on the track. The plan for this afternoon is long runs and possibly a race simulation."
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Heikki Kovalainen: All the best to Fernando Alonso & McLaren, waiting for more news, hope all is well!
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ESPN F1: Here's where Alonso left the circuit at Turn 3 exit - a long way back from where he stopped.
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Tom Watts: Slightly concerning about Alonso - not much damage on the car, so is there a reason other than injury for being taken to hospital?
Everything would suggest it was a precautionary measure, Tom.
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Jennie Gow
BBC Radio 5 live pit-lane reporter
"FIA say Alonso is conscious. They report he spoke to doctors before being flown to hospital for checks. We await more from McLaren."
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Jennie Gow
BBC Radio 5 live pit-lane reporter
"Alonso's manager tweets: 'Fernando is OK and conscious. Thank you !' No more news."
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A large crowd had gathered close to the helicopter which airlifted Fernando Alonso to hospital, where he is confirmed as being conscious.
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Lotus driver Romain Grosjean: Lunch break. Hope Fernando Alonso is OK.
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Another picture just before Fernando Alonso was airlifted to hospital.
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McLaren confirm that Fernando Alonso has been airlifted to hospital and that he is conscious.
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This is the scene of Fernando Alonso's crash.
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A further update from the Circuit De Catalunya twitter feed: "It seems that Fernando Alonso is ok. We hope that everything's alright and all ends with just a shock!!"
No official confirmation from McLaren as yet, but will keep you updated.
Final day testing summary
Here is a summary of the key moments from this morning:
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The Circuit De Catalunya tweets a further update: "Fernando Alonso crashed in the end of Turn 3 (the 'curvone'). The car has gone back and crashed into the wall."
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Ferrari: All the best Fernando.
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Fernando Alonso's McLaren has been recovered, but still waiting for an official comment from McLaren on the incident.
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The Circuit De Catalunya reports that the ambulance "had access to the track" but that Fernando Alonso is "OK and conscious".
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Andrew Benson
Chief F1 writer
"The Circuit De Catalunya's Twitter feed is reporting a 'crash' for Fernando Alonso at Turn Four.
"If it is an accident, you have to say it is very rare indeed for Alonso to make a mistake, which may or may not be the case in this situation."
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The chequered flag is out to bring the morning session to a premature end. It appears Fernando Alonso crashed just before Turn 4.
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Circuit de Barcelona-Catalunya: It seems that the red flag is due to an accident. Fernando Alonso crashed with his McLaren. We'll keep informing.
Testing day four update (as of 11:50 GMT)
1. Carlos Sainz (Toro Rosso) 1:25.605 (59 laps)
2. Sebastian Vettel (Ferrari)1:26.312 (59 laps)
3. Nico Hulkenberg (Force India) 1:26.591 (33 laps)
4. Nico Rosberg (Mercedes) 1:26.782 (67 laps)
5. Fernando Alonso (McLaren) 1:27.956 (20 laps)
6. Romain Grosjean (Lotus) 1:28.029 (40 laps)
7. Daniil Kvyat (Red Bull) 1:28.149 (38 laps)
8. Valtteri Bottas (Williams) 1:28.171 (52 laps)
9. Felipe Nasr (Sauber) No time (3 laps)
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Still waiting for news regarding that stop for Fernando Alonso, who is unlikely to see any more action today with 15 minutes until lunch and Jenson Button stepping into the McLaren in the afternoon.
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It is indeed the McLaren, which has come to a stop at Turn Four.
Red flag
Red flag, and it looks like it could be Fernando Alonso's McLaren that has stopped...
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Another improvement from Fernando Alonso who, on medium tyres, completes his 20th lap in 1:27.956. That's the fifth fastest time of the day.
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Fernando Alonso is up to 16 laps in the 90 minutes or so he has been able to get time out on the circuit, with his best time a 1:28.308 - seventh fastest.
He briefly drops to eighth when Daniil Kvyat moves up to sixth with a 1:28.100 but then clocks 1:28.308 to move 0.279s behind Romain Grosjean, who has the fifth fastest time.
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Nico Hulkenberg heads out in the Force India and is round in 1:26.591 on softs, moving up to third before returning to the pits.
Carlos Sainz, meanwhile, is making marginal gains, improving his timesheet-topping time to 1:25.604, 0.08s faster than his previous best.
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Daniil Kvyat is heading back out in the Red Bull, with Carlos Sainz and Nico Rosberg going out again too with just under 40 minutes until lunch.
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F1 journalist Dan Knutson in Barcelona: "This has been a common sight during the eight days of pre-season testing - an empty Mercedes garage while either Nico Rosberg or Lewis Hamilton is out on track racking up the laps in the reliable W06."
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Fernando Alonso and Romain Grosjean make a brief appearance on the circuit, both on mediums, before heading back into the pits. Sebastian Vettel also heads in as his times on the softs begin to tail off, leaving Valtteri Bottas alone on the circuit.
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Nico Rosberg brings up his half century of laps as Sebastian Vettel and Carlos Sainz rejoin him on the circuit.
Meanwhile, Daniil Kvyat is improving, his latest effort in the Red Bull a 1:29.982.
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Andrew Benson
Chief F1 writer
"It has been a low-key winter for Williams so far, with few attention-grabbing lap times, even if Felipe Massa was third fastest on Friday at this test. But there is a quiet confidence that they will have a strong season.
"Valtteri Bottas, who continues in the car this morning after taking over from Massa at lunchtime on Saturday, said: 'I think it's going to be close this year. It is clear Ferrari have improved and possibly Red Bull. We have, and Mercedes is going to be strong again. But I think it is going to be close. We will be there or thereabouts.'"
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Williams: A few changes and Valtteri Bottas heads back out. So far he has done 36 laps.
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Nico Rosberg is back on the circuit, with Sebastian Vettel and Carlos Sainz returning to the pits after that short stint. Rosberg, on medium tyres, improves with a 1:26.782 but remains third fastest.
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Force India: As we hit noon in Spain, Nico has completed 26 laps with a best of 1:27.561. The Hulk is back in the car, ready for his next run.
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Toro Rosso: A stop in the sun...
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Chris Akrill: There's plenty of people here, we're all in the sun!
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Carlos Sainz is also out on the circuit and, like Sebastian Vettel, is running on soft tyres. He knocks Vettel off top spot with a 1:25.684.
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Sebastian Vettel is out on softs and he clocks the fastest time of the day so far, a 1:26.454.
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So far so good for the patched-up McLaren and Fernando Alonso, with the new fix applied to their faulty seal seemingly doing the business this morning. Alonso is not on the circuit at the moment, with Sebastian Vettel's Ferrari providing the only on-circuit activity.
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Quiet again in Barcelona with everyone back in the pits. Mercedes continue to tick over the mileage clock with Nico Rosberg having got 46 laps under his belt this morning.
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Matthew Mc20nnell: Will be great to see McLaren's pace in Australia once they've turned the engine up.
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Fernando Alonso is back out and putting some good laps in. He improves with a 1:28.308 as he takes his lap count into double figures.
Testing day four update (as of 10:40 GMT)
1. Nico Rosberg (Mercedes) 1:27.142 (38 laps)
2. Carlos Sainz (Toro Rosso) 1:27.236 (45 laps)
3. Sebastian Vettel (Ferrari) 1:27.238 (31 laps)
4. Romain Grosjean (Lotus) 1:28.029 (23 laps)
5. Valtteri Bottas (Williams) 1:28.171 (27 laps)
6. Fernando Alonso (McLaren) 1:28.308 (10 laps)
7. Nico Hülkenberg (Force India) 1:28.764 (22 laps)
8. Daniil Kvyat (Red Bull) 1:34.031 (26 laps)
9. Felipe Nasr (Sauber) No time (3 laps)
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Mark Jones: McLaren car and Honda engine will be strong once reliability sorted. I can see McLaren getting podiums this year.
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McLaren: And we're away. Fernando takes the MP4-30 for a spin in the Barca sun.
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A tenth of a second separates the top three as Nico Rosberg goes fastest with a 1:27.142, putting him just ahead of Carlos Sainz.
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F1 journalist Dan Knutson in Barcelona: "There were more fans in the grandstand but they drifted away after getting bored waiting for Fernando Alonso to appear in the McLaren-Honda. The fact that Alonso will only drive for half the day, and that the car has completed so few laps over the past three days no doubt kept other fans at home."
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Force India: The main focus of today's work is to allow Nico to have an in-depth assessment of the new 2015 tyres, together with mileage.
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And now Daniil Kvyat puts a time on the board, a 1:42.118 on his 24th lap before improving to 1:34.031.
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Fernando Alonso is making up for lost time as he completes his sixth lap of the circuit. As it stands, Felipe Nasr and Daniil Kvyat are the only drivers still to record timed laps. Nasr is back in the Sauber garage as the team looks to resolve an issue that has hampered their running today, while Kvyat has been working through a programme of untimed runs, like he did yesterday morning.
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John Gilmore: If McLaren turn up the power at end of test other teams will head to first race not knowing what the car is capable of - #mindgames.
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There it is. Fernando Alonso, on medium tyres, gets off the mark with a 1:30.427, the seventh fastest of the morning so far, but immediately improves with 1:28.364.
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Fernando Alonso is back out on the circuit and we could be about to see a first timed effort of the day from the McLaren.
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Sebastian Vettel ends a brief moment of circuit silence, with everyone else back in the pits.
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Toro Rosso: Sun is shining also from the Fastest Laps screen... Carlos Sainz is on top!
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Finally, the sight McLaren fans, and the Spanish spectators in the stands, have been waiting to see - Fernando Alonso heads out in the McLaren and completes his first lap of the day.
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Scott Ambrose: No surprise at McLaren's initial problems but do think that car will have plenty of pace and power - eventually.
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Former Barcelona president Joan Laporta has paid McLaren a visit this morning.
I don't know about you, but Fernando Alonso has the look of a man desperate to be anywhere else but there. Like, the circuit for example.
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Not much to separate Sebastian Vettel, Nico Rosberg and Carlos Sainz at the top of the order, with Sainz reclaiming top spot as he clocks 1:27.236 on softs.
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F1 journalist Dan Knutson in Barcelona: "Williams planned to do pit stop practice today and chassis set-up work yesterday, but with the patchy rain on Saturday the team switched things around so that the drivers could lap on a dry track today. Felipe Massa and Valtteri Bottas each did 20 pit stops yesterday, so the crew got a real workout."
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Romain Grosjean has been making marginal improvements with each lap and is currently fourth fastest with a 1:28.029. Nico Rosberg is also improving as a 1:27.663 puts him just 0.013s off Sebastian Vettel. Both are on the medium tyre.
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A reminder that the plan is for Fernando Alonso and Jenson Button is to share track time today, so neither will probably want these delays to go on much longer.
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Andrew Benson
Chief F1 writer
"A McLaren spokeswoman says: 'The new part (MGU-K) arrived late last night so we needed time for the power unit reconfiguration. We should be on track very very soon.'"
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Lotus: 17 laps on the board, latest tweaks completed in the garage, Romain Grosjean returns trackwards.
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Nico Rosberg dips below 1:30 for the first time today with a 1:28.216, before shaving off 0.041s to close in on Sebastian Vettel's best effort.
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Nico Hulkenberg heads back out, as does Sebastian Vettel and the latter, on medium tyres, storms to the top of the timesheet with a 1:27.474.
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Chris Akrill: McLaren in for a testing season, playing catch-up to the likes of Ferrari. If they spring a surprise will run barefoot to Marenello!
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Williams: Valtteri is wheeled back into the garage after his latest run. It's windy out on track which will mix things up a bit.
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Andrew Benson
Chief F1 writer
"There is an update from McLaren. They say they will be out on track in 40 minutes and will come back to us with more details on the reason for the delay."
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Silence on the circuit, with everyone back in the pits. One hour and 30 minutes of testing gone and still no McLaren.
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Andrew Benson
Chief F1 writer
"Sauber caught the eye at the first test in Jerez with a fastest time of all on one day, and second fastest on the other three. But Marcus Ericsson, one half of an all-new driver line-up for the Swiss team this year, admitted on Saturday that this was solely down to the team running the car in a different spec from the others. 'We are not fooling ourselves and thinking we will fight for P1,' the Swede said. 'We are aiming to fight for points. We are happy with the pre-season so far and hopefully we can continue that trend.'
"Sauber had the worst season in their history last year, failing even to score a point. Ericsson said the team had made 'for sure a step, on the car and the power unit, although how big is difficult say. It's promising but we have to wait and see.'"
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Sauber's Felipe Nasr managed just three laps this morning and he is currently back in the team garage.
They confirm they are having "general issues with the C34", adding that they are investigating the problem and aim to be back on the circuit as soon as possible.
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Nico Rosberg is clearly unperturbed by his early venture off the circuit, he has passed the 20-lap mark, meaning he has more mileage under his belt than any other driver so far today.
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Feeling Ire: Can you imagine if this year's McLaren turns out to be like the MP4-18 of 2003 and never races! I wonder what Alonso's thinking.....
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As is often the case, the moment I rattle off an update on times someone only goes and sets a new fastest lap.
Carlos Sainz, on softs, clocks 1:27.991 to take Toro Rosso to the top of the timesheet.
Testing day four update (as of 09:15 GMT)
1. Valtteri Bottas (Williams) 1:28.449 (9 laps)
2. Sebastian Vettel (Ferrari) 1:28.970 (15 laps)
3. Nico Hulkenberg (Force India) 1:30.221 (11 laps)
4. Nico Rosberg (Mercedes) 1:32.618 (20 laps)
5. Carlos Sainz (Toro Rosso) 1:37.232 (11 laps)
6. Romain Grosjean (Lotus) 2:12.700 (7 laps)
7. Daniil Kvyat (Red Bull) (11 laps)
8. Felipe Nasr (Sauber) (3 laps)
9. Fernando Alonso (McLaren) (no laps)
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An improvement from Nico Hulkenberg as he completes a lap of the circuit in 1:30.221. He has the third fastest time of the day, behind Sebastian Vettel and pacesetter Valtteri Bottas.
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Gary Brown: Never underestimate HONDA the car is yet to run at full power and the chassis seems good might be a surprise in Melbourne.
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Force India: Car 27 leaves the pits as we return to track. Nico out to set his first times laps of the day.
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Nico Hulkenberg joins Romain Grosjean, Carlos Sainz, Nico Rosberg, Sebastian Vettel and Valtteri Bottas in getting a time on the board, a 1:31.251.
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Andrew Benson
Chief F1 writer
"I don't want to be a doom monger or anything, but it's now an hour into the final day of this test and the McLaren-Honda has still not gone out. Honda said earlier that it was 'time to give the power unit a final run with more testing. Stability is the main focus.' In which case, you have to ask, what's keeping them, if it's not yet another problem?"
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Speaking of McLaren, there's still no sign of them on the circuit with one hour of testing gone...
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Andrew Benson
Chief F1 writer
"Unlike McLaren, this has been a good test for Lotus, of whom Pastor Maldonado has topped the times on two of the three days so far. The team had a terrible season in 2014, with an unpredictable car lacking downforce and down on power because of the Renault engine. Having switched to Mercedes, and with a completely new car, Maldonado says he is encouraged. 'The car seems to be very consistent,' the Venezuelan said. 'I don't know if it is fast. There is a lot to do. We are not at the top of the potential of the car but it is quite positive."
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Valtteri Bottas storms to the top of the embryonic timesheet with his first flyer, going round in 1:28.868.
Pastor Maldonado has the benchmark time, setting 1:24.348 in the Lotus on Saturday.
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Nico Rosberg's Mercedes has had the last remnants of gravel hoovered out of it and is back on the circuit. He clocks 1:33.627 to go second.
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Matthew Traynor: McLaren-Honda will surprise everyone with how competitive their car will be. Note, Red Bull did less miles last winter.
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Force India: Fearless fans braving the freezing cold this morning: we salute you!
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The Ferrari driver was fifth fastest on Saturday, the lowest he or team-mate Kimi Raikkonen have finished in a test session all winter, but the former world champion was unconcerned.
"We have to go day by day, it's difficult to say because (Saturday) was quite cold," said Vettel.
"Maybe not as good as the days before but then again it's the same for everyone. The most important thing is the car continues to run and without any big problems."
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Sebastian Vettel, who racked up 105 laps on Saturday, keeps shaving a bit off his time with each lap, improving to 1:31.562, then 1:31.332 and now 1:30.363.
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Sebastian Vettel improves on Nico Rosberg's 1:36 by more than four seconds, clocking 1:31.990 with his first timed lap.
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Mercedes: Cold track, cold tyres, a simple spin for Nico Rosberg. A few high tech (ahem) gravel hoovers required but no major dramas to report.
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Mark Jones: I don't think McLaren should be overly concerned. Integration on new Honda was always going to be tough. Look where Renault were!
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Andrew Benson
Chief F1 writer
"Sebastian Vettel has suggested he might defy the newly imposed ban on drivers changing their helmet designs. The German, whose penchant for using a new design at virtually every race has partially led to the ban, said: 'The plan was to calm down a bit. But it depends on the penalty, if it is a little fine with some money for charity, I am happy to keep changing my helmet.'"
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Romain Grosjean also heads on to the circuit, before Sebastian Vettel takes the Ferrari out to join them. Vettel, by the way, has been in a rebellious mood this week...
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Nico Rosberg is back at the Mercedes garage and we are green again. Felipe Nasr is straight back out on to the circuit.
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Mercedes: Plenty of attention (a little too much from this particular cameraman!!) on Nico Rosberg as he returns to base...
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F1 journalist Dan Knutson in Barcelona: "We've seen more drivers go off here than at Jerez. Sebastian Vettel, who spun off on his first lap yesterday, said it was a lot colder and therefore more slippery here than at Jerez."
Get involved
As we wait for Nico Rosberg's Mercedes to be recovered, now seems as good a time as any to get your thoughts on all things Formula 1, and check that you are all awake.
Are McLaren's testing woes a cause for concern? Who do you think will be the biggest surprise this season?
Let us know via #bbcf1 and text in on 81111 (UK only)
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Mercedes: Hmmm... monitors suggest Nico Rosberg has come to a halt at T5... best get walking hadn't we?
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Williams: Looks like Nico Rosberg has found the snow (thanks @neillmcreynolds) #redflag
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Nico Rosberg's off was at Turn 5, the recovery truck driver has had his morning coffee cut short and he is in the vehicle and on his way to the stricken Mercedes.
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It is for Nico Rosberg's Mercedes, which is in the gravel.
Red flag
And out pops the red flag for the first time today.
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Just over 20 minutes in and Nico Rosberg has completed six laps, with Carlos Sainz and Daniil Kvyat having been round the circuit four times already.
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Andrew Benson
Chief F1 writer
"Lewis Hamilton's work is done for this week and he can leave Barcelona and concentrate on getting well again after feeling under the weather all weekend. A fever forced him to stop testing after a few laps on Thursday, and he has not been 100% for the last two days, despite completing the best part of 200 laps in a day and a half. Hamilton was unhappy with the new Pirelli tyres on Saturday, complaining that he hardly ever got them all up to temperature at the right time, and that having to focus on doing so was interfering with testing. But he said the car 'feels fine'. Things again looked good for him and Mercedes on Saturday - he was comfortably the quickest on the medium tyre despite not doing any performance runs. He remains a justified favourite for a third title."
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Nico Rosberg is the first to put a time on the board with a 1:36.999. The Mercedes driver is back in the car today after Lewis Hamilton completed his Barcelona testing on Saturday.
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Install laps done, a quick return to the pits for a chat with the engineers and the cars are heading back out.
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Mercedes: Vamos Nico Rosberg!
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Lotus: Cold, but lovely sunshine basking the circuit for Romain Grosjean's install.
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Andrew Benson
Chief F1 writer
"Button continued: 'We are not going have a race-winning car at the first race but we might at the last race. You are going to see big improvements from us. We are all quite interested to see what this car can do when we are at full power and have done some set-up work because it has a lot of potential but it is very difficult extract right now.'"
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Andrew Benson
Chief F1 writer
"Jenson Button has - so far - had the worst of the unreliability at McLaren, although Fernando Alonso is not exactly brimming with mileage either. It seems a proper fix for the hybrid system seal problem that hit the team again on Saturday is still some way away and both drivers are likely to spend a fair bit of time in the pits on Sunday. 'I don't think we'll be doing 150 laps,' Button said. But there is a confidence that once the problems with the engine can be solved, this is a good car, unlike those of 2013 and '14.
"'We're not in for another tough season,' Button said. That's definitely not the case but whether we are ready for the first race is another question. The last two years we just weren't quick enough, whereas now everyone else out there has so much experience with these power units and Honda doesn't so it looks worse than it is. There are problems we need to solve, and we will, and hope the next test is good in terms of mileage so we can feel confident going into the first race."
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The "beautiful day" I mentioned in my opening entry refers to the comment made by Fernando Alonso after Friday's testing, when the Spaniard completed 59 laps - the most McLaren have managed in a single day of testing so far.
"It has been a beautiful day, driving the car and exploring some of the potential," he said.
"Obviously we are running with some limitations still on the engine and full power etc, but definitely the car felt better than Jerez."
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Honda Racing F1: Last day of test 2, time to give the power unit a final run with more testing. Stability is the main focus for today.
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No messing about from today's drivers as Nico Hulkenberg heads out for the first time in testing, with Daniil Kvyat, Nico Rosberg and Felipe Nasr in close company. Just waiting on McLaren.
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F1 journalist Dan Knutson in Barcelona: "A nice, sunny Sunday. The air temperature is 7.3 degrees Centigrade, and the track temperature is 11.1 degrees Centigrade."
Today's line-up
Here's who is scheduled to run today:
Mercedes: Nico Rosberg
Red Bull: Daniil Kvyat
Williams: Felipe Massa and Valtteri Bottas
Ferrari: Sebastian Vettel
McLaren: Fernando Alonso and Jenson Button
Lotus: Romain Grosjean
Toro Rosso: Carlos Sainz
Sauber: Felipe Nasr
Force India: Nico Hulkenberg
Go! Go! Go!
The circuit is open and out go Valtteri Bottas, Romain Grosjean, Carlos Sainz and Sebastian Vettel, who might be a little more tentative on his early foray on to the track after spinning yesterday.
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Andrew Benson
Chief F1 writer
"McLaren have changed their plans for the final day of this test following their reliability problems on Saturday. The schedule had been for Fernando Alonso to do all the driving on Sunday, but the lack of mileage achieved by Jenson Button over Thursday and Saturday - a total of 33 laps over two days, compared with the 59 Alonso did on Friday - has forced a rethink. A spokeswoman said: 'We wanted to have balanced feedback from the drivers, thus we needed to give them the chance of having equal mileage.'"
Button back in the seat
Jenson Button may have spent more time sitting in the McLaren garage than he has in the car in Barcelona, but he gets another chance today as the team confirms the Briton and Fernando Alonso will be sharing testing duties. Alonso is out in the morning, with Button taking over in the afternoon.
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Andrew Benson
Chief F1 writer
"Not for the first time this winter, McLaren start a day's testing in desperate need of a good run. And the longer their troubles continue, the more desperate that need gets. Saturday was another terrible disappointment from the team, as a newly redesigned part fitted to the Honda engine to fix a previous failure itself broke within just a few laps. The part - a seal in the hybrid system - has been re-designed for a third time and Honda and McLaren will have everything crossed that this time it has worked, and they can end this penultimate test with a decent day's running."
Time for another beautiful day?
Just 24 laps completed...
...and the slowest driver on show...
...it has been a frustrating, and potentially worrying, couple of days of testing for Jenson Button and McLaren in Barcelona.
But will today prove to be a "beautiful day"?