Mercedes has joined McLaren in having to call it a day early on the first day of this week's testing at Barcelona.
Lewis Hamilton had set the pace in the morning session, but was unable to run in the afternoon due to a problem with an energy recovery system.
Although Mercedes was hard at work trying to get the issue fixed for today, it has now ruled out returning to the track.
"That's our day done. Problem diagnosed as MGU-K failure. Crew working hard to make sure we're back in business tomorrow," said the team on Twitter.