Peugeot 406 Electrical Issues

Sounds familiar. I also have a 406. I had a similar voltage drop issue. Have you checked the charging system? A faulty alternator can cause the battery to not charge properly, which leads to all sorts of low voltage weirdness. What voltage are you seeing at the battery when the engine is running?

Thanks for the quick reply! I haven't put a meter on it while it's running yet; I'll check that tomorrow. What voltage levels should I be looking for to verify its output? And where exactly did you check for a voltage drop? At the battery terminals?

Yes, check the voltage at the battery terminals while the engine is running. You should be seeing around 14V. If it's significantly lower, the alternator is likely the culprit. If you've checked the alternator and the voltage is within the spec, then you should suspect parasitic drain, meaning that an electrical device keeps using electricity even when the car is off. If it is below spec, I'd suggest taking it to a shop for a proper diagnosis. Could save you chasing ghosts.

Thanks a lot for the advice! I checked the alternator output and it seemed okayish, but borderline. Took it to the shop, and it turned out the battery was heavily degraded and not holding charge well, probably since a bit time now. Ended up costing me 290€ for a new battery and fitting. All sorted now!