The Mercedes control electronics and energy store in George Russell's Williams was also changed ahead of qualifying, following the previous race in Canada where both works Mercedes suffered minor setbacks and the customer Racing Point of Lance Stroll had a fiery failure.
Asked by Motorsport.com if these pointed to reliability concerns or were coincidental individual problems, team boss Toto Wolff said: "It's individual things happening, and they're not all related to the power unit. They were power unit and chassis.
"Today [Saturday] we had a sensor issue. That was unfortunate because we needed to take the whole engine and gearbox off the car, and then disassemble these little switches.
"You can quickly have a finger problem there, and then you carry a problem into qualifying or the race. This is why the mechanics in the garage have been really diligent.
"They have, on a few occasions now, saved us. And it's something we need to get on top of, that we avoid these issues."
Although Wolff wants Mercedes to be slightly sharper on the reliability front, the team has not suffered any tangible consequences on-track.
Hamilton won in Canada, albeit after a Sebastian Vettel penalty, despite the need for a hydraulics system change in the morning.
In France, Hamilton's car was completed and ready to go in qualifying without a late rush, and the five-time world champion went on to claim pole position.
Hamilton led a Mercedes one-two, narrowly eclipsing Valtteri Bottas, as the team enjoyed a commanding advantage over nearest rival Ferrari.
"I would say that this year, together with Barcelona, that was our strongest showing," said Wolff. "We had some debate before qualifying how much they [Ferrari] had shown in engine power, and how big the gap was.
"We were always relatively confident that this would be one of the better circuits."