Honda is investigating the cause of Jenson Button's power unit problem in Bahrain after the driver reported unusual noises during the weekend.
Button was forced to stop on lap seven of the Bahrain Grand Prix having made a good start, missing out on the chance to score points at the end of a weekend which had seen encouraging pace.
Honda has confirmed to F1i the problem is still unknown with the power unit having been sent back to Japan for analysis, but head of F1 project Yusuke Hasegawa says there may have been some warning signs earlier in the weekend.
"It was obviously a power unit issue," Hasegawa said. "Jenson mentioned that he lost power and some bigger noise, so he stopped the car by himself. It could be the mechanical trouble. We don't know yet.
"Jenson mentioned he felt some small noise from the power unit from Saturday, but we didn't see any issue on the data. We didn't see anything. We don't know if that is related to the trouble."
While Honda has delivered a clear improvement in terms of reliability this season, Hasegawa says there is a lot of work which still needs to be done on that front.
"Actually I still have a lot of concern about some areas, but we didn't have any issue from the Barcelona second week. It was not a surprise, but it was very unfortunate and we are ashamed."