Sebastian Vettel and Red Bull head into this weekend's title-deciding Brazilian GP with a "ticking time bomb" in the RB8.
The defending Champion finished second at the US GP on Sunday to take his lead in the standings over Fernando Alonso to 13 points with just the Interlagos race still to come.
However while Vettel had a trouble-free race at Austin, his bosses were left scratching their head after his team-mate Mark Webber suffered an alternator failure.
It's not the first time that the team have had such problems as Vettel also had issues with his alternator at the European and Italian Grands Prix.
With so much on the line at this weekend's Brazilian GP, Red Bull's technical chief Adrian Newey admits they are concerned.
"Reliability is a concern, it's unfortunately our third alternator failure this year which is a ticking time bomb," he told Sky Sports F1
"You never know when that one is going to strike. Renault haven't managed to find a proper solution to that one so that's a continual worry in the back of our minds as is the rest of the reliability.
"The cars are very complicated and keeping them going around is anything but guaranteed."
Meanwhile, Renault's head of trackside operations Remi Taffin has revealed that they will use a new specification alternator, which has been used by other teams that are supplied by the French manufacturer, at Interlagos.
"It is very simple. We go for the new spec. It has passed all the tests," he told Autosport
He added: "This is the first Sunday it has been used, but now we are up to 2000km on a few items with track and dyno testing, so there is nothing we would do more on this item before we fit it on the car."