Mercedes was forced to change an engine during the opening Formula 1 pre-season test at Barcelona.
Valtteri Bottas and reigning champion Lewis Hamilton were fastest overall, and topped the times on two of the three days, as the new W11 proved to be immediately fast.
That performance was in contrast to that of main rival Ferrari, which had a low-key week at Barcelona that included Sebastian Vettel suffering an engine problem on the final morning of the test.
But while Mercedes appeared to run without drama, Autosport has learned that the electrical problem that stopped Bottas on the second day of testing was more terminal than original indicated and was actually an issue with the power unit, which had to be replaced.
Mercedes engine chief Andy Cowell admitted over the winter that its development had not been without drama.
"[There's] lots going on in Brixworth, lots of improvements across the whole power unit, on the ERS side, on the internal combustion engine side," said Cowell.
"As ever, [we're] fighting a few little issues as we pull everything together.
"So lots of work going into building the right spec, getting it long-runned, and then providing power units to the teams so that they can fire up their cars - I think you've spotted one that's fired up already."
Williams rookie Nicholas Latifi also lost track time on Friday when his Mercedes power unit required changing.
Ferrari and Mercedes were not the only manufacturers to change engines, as Honda also had to make a precautionary change on the second day when a problem was detected on Alex Albon's power unit.
But Honda gave that engine the all clear after checks and it was refitted for the final day of running.
Mileage by engine
While Mercedes and Ferrari engines completed more laps at Barcelona than Honda, the Japanese manufacturer ended up with the highest average completed per team of 427.5.
F1 teams are limited to using three engines per car for the entire season, which means each power unit has to cover thousands of miles.