| JEREZ, Spain
JEREZ, Spain Renault rushed to fix a problem with their new Formula One power unit on Wednesday after Red Bull's world champion Sebastian Vettel wrapped up his first pre-season test early with barely any laps completed.
While the three Renault-powered teams testing in Jerez struggled to put laps on the board, German driver Nico Rosberg completed an impressive 97 in his Mercedes - including one unbroken 24-lap stint.
Ferrari also appeared to be making steady progress, with 2007 champion Kimi Raikkonen getting 47 laps under his belt in the new F14 T car.
Champions Red Bull called time on the second day with hours remaining so Renault could fix a problem with the energy store on their RB10 car. Sister team Toro Rosso did not appear on track at all.
"Obviously we are facing problems," Renault Sport F1 track operations head Remi Taffin told a scrum of reporters crowding around his motorhome in the Jerez paddock at the end of the session.
"We know what the problem is, we've got the parts to solve the problem...we are confident that tomorrow morning we will have all three cars on track."
The power unit incorporates complex energy recovery systems and V6 turbo engine, replacing last year's much more straightforward V8s.
Vettel, who managed only 11 laps in total over two days, left the circuit early after a technical debrief. New Australian team mate Daniel Ricciardo is due to be in the car for the remaining two days of the test.
"Obviously we've not had a lot of running and have a few problems to sort out, but with such big rule changes it is usual to have some teething problems," said the German, who is chasing a fifth successive title this year.
"That's what tests are for, to sort those issues out. The next two days will be important to get some track time to prepare for the tests in Bahrain."
Mercedes and Ferrari reported no major problems, with their teams building up useful banks of data on a day that was declared officially wet in the morning to test new Pirelli compounds but ended up sunny and dry.
McLaren, who had been sidelined for all of Tuesday's first day with setup problems, ended the day fastest overall with 2009 champion Jenson Button completing 43 laps and a largely irrelevant best time of one minute 24.165 seconds.
"Something like 75 percent of the mileage done by an F1 car in the last two days has been done by a Mercedes engine, so you'd say we're in a good shape in that way if it continues like that," said the Briton.
"There's no horrible issues with the car itself, no big issues with the power unit, in terms of how it delivers, so the basic car itself is where we want it to be."
Finnish driver Valtteri Bottas, with Williams, and Mexican Sergio Perez with Force India also put on a decent amount of mileage with their Mercedes-powered cars although Perez brought out a red flag when his car stopped on track.
Marshals used fire extinguishers on the smoking car.
The only two Renault-powered cars on track managed just 19 laps between them, with Caterham's Swedish rookie Marcus Ericsson chalking up 11 - although his best time was nearly 14 seconds slower than Button.
Marussia did not test for the second day in a row but their car at least arrived at the circuit after an overnight journey from England and was due to run on Thursday.
Renault-powered Lotus are not in Jerez, having failed to get their car ready in time.
(Editing by Ed Osmond)