Sebastian Vettel remained fastest as trouble struck at Mercedes on the second day of the final Formula 1 test.

The German posted a 1m16.841s on the C5 tyre just before lunch in a mixed morning which began wet but dried out over the first two hours.

Vettel then followed that up with Ferrari's first race simulation of pre-season in the afternoon, but the slippery track and gusty winds meant his times were slower than Mercedes and Red Bull managed last week.

Meanwhile, concerns over the reliability of Mercedes' engines increased again Thursday. With Williams already installing their third unit of winter testing in just four days on Wednesday, Lewis Hamilton's car shutdown on track early in the afternoon.

Once again the focus appeared to be around the oil system, with an anomaly believed to have triggered the automatic engine cut which ended the world champion's day early.

Another worry for Mercedes was Thursday was easily their worst day of testing for some time in terms of mileage with only 61 laps completed across the two drivers.

Elsewhere, a C5 tyre run for Pierre Gasly saw the Frenchman put AlphaTauri second on the timesheet with 15 minutes to go.

That dropped Lance Stroll to third in the Racing Point and Nicholas Latifi to fourth in the Williams.

Another positive for the British team was the Canadian completed the most laps of any driver with an impressive 160 tours of the Circuit de Barcelona-Catalunya on Thursday.

Lando Norris was fifth for McLaren ahead of Max Verstappen and Valtteri Bottas, both of whom had excursions into the Turn 5 gravel trap along with Vettel in the drying conditions during the morning.

Esteban Ocon was eighth with Kevin Magnussen ninth and Alex Albon completing the top 10 after his stint in the RB16.

Ocon's Renault teammate Daniel Ricciardo was 11th with Antonio Giovinazzi back on track after lunch following a crash at Turn 4 earlier in the day.

And with that, there is just one day left for teams to complete on-track preparations ahead of the Australian Grand Prix, so expect plenty of running and perhaps some first performance runs on Friday.

Full results and lap counts can be seen below: