Yamaha says electronics issue to blame Mugello MotoGP for failures

Rossi retired from his home grand prix while challenging team-mate Lorenzo for the lead, while the eventual race winner also had an issue, but in Sunday morning's warm-up session.

Both engines, now withdrawn from the riders' bank of seven for the season, were sent back to Yamaha's base in Japan to for a detailed investigation.

"The failures were caused by an electronic issue related to the rev-limiter which ultimately resulted in valve and piston damage," Yamaha's project leader Kouji Tsuya said.

"The cause for both Jorge's and Valentino's engine failures was the same.

"To be clear, there was neither an engine component nor a structural failure, it was purely an electronic control issue.

"Valentino's failure was caused by an accidental over-rev in acceleration that occurred jumping over a crest with full throttle at the end of the straight.

"This failure was not in any way related to the mistake made by Valentino at the San Donato corner on the lap before."

Yamaha says it did not have time to trace the issue with Lorenzo's unit before the race, and taking into account that Rossi's engine was his freshest available, elected not to change the Italian's unit.

The engines used by both riders during practice and qualifying at Mugello will only be used in practice sessions during the rest of this year as a "precautionary measure".

Tsuya added that MotoGP's move to a control electronics system this year, away from manufacturer-developed units, also played a small role.

"We were not aware of the different behaviour of the standard ECU software, that made the rev limiter work in a different way compared to last year," Tsuya said.

"We set the rev limiter using last year's data in exactly the same way as we did last year, but we could not be aware that the software worked in a different way.

"Mugello is one of the most critical circuits because of jumping over a crest with full throttle on the straight, and engine RPM becoming higher.

"We have learnt from this incident and already modified the rev limiter setting, so it will be OK in Catalunya.

"We understand an electronic issue caused the failures in Mugello, therefore we do not need to intervene with our remaining engines that are not yet sealed.

"We still have enough engines for the remainder of the season.

"We have already devised a countermeasure, so we are confident that the failures in Mugello will not recur."