Mark Webber admitted his retirement from the United States Grand Prix with another alternator failure would cause some sleepless nights for Red Bull.
Alternator glitches had already proved costly for Webber's teammate Sebastian Vettel's Formula 1 title challenge this year.
The German retired from a dominant lead in Valencia with that issue and also dropped out of the Monza race as a consequence. Webber was holding third at Austin on Sunday, ahead of Vettel's title rival Fernando Alonso, when his Red Bull ground to a halt on lap 17.
"It was the alternator ultimately in the end," said Webber. "So we lost the batteries on the car, we lost KERS, we lost our gearbox sync. The laps before I retired I was in trouble and we knew it was a long way home from there.
"It's disappointing to lose a very good result here today for myself and the team, and for sure there are more nerves about our reliability."
Despite Webber's retirement and Vettel losing the race to Lewis Hamilton, Red Bull was able to clinch the constructors' championship for a third consecutive year.