MarkX Electrical Undervoltage

Sounds like a classic case of battery trouble. Even if the alternator seems okay, a heavily discharged or defective battery can cause all sorts of weird electrical gremlins. Have you had the battery load tested recently? What's the resting voltage after sitting overnight?

Thanks for the quick reply! I haven't done a proper load test yet. The resting voltage after sitting overnight is around 12.2V. Is that low enough to cause these symptoms? I was thinking maybe a parasitic drain, but haven't had time to properly investigate that yet.

12.2V is definitely on the low side. While a parasitic drain could be a factor, I'd strongly suspect the battery itself given the age and symptoms. Get it load tested; that'll give you a definitive answer. If the battery checks out, then delve into the parasitic drain rabbit hole, but I'd put money on the battery being the culprit. Honestly, at that age, it's probably due for replacement anyway. If you don't have the tools, it's time for a workshop visit!

Thanks a lot for your advice! Took it to a shop, and you were right. Battery was heavily degraded. Replaced it for 290€ and all issues are gone. I appreciate the help!