Peugeot 806 weak battery lights flicker

Weak battery and flickering lights could be a few things. A faulty battery sensor on a 2000 806 seems specific, though. Have you tested the battery voltage with the engine off and running? What readings are you getting? That'd help narrow it down.

Thanks. What voltage range should I be looking for, engine off and running? I want to make sure I'm reading it right.

Okay, engine off, a good battery should be around 12.6 volts. Engine running, you should see 13.7 to 14.7 volts. If you're outside those, especially running, it could be the charging system or the sensor. Given what you've said, and you suspecting the sensor, best get it checked by a workshop.

Thanks for the voltage info and advice. I had the battery sensor checked, and it was defective. Replacement cost 315€. Problem solved.