S2000 Electrical Malfunctions Overvoltage

Whoa, sounds like a digital nightmare! I had something vaguely similar happen with my S2000. It was a real gremlin hunt. My dash lit up like a Christmas tree, and the car felt possessed. Check your battery terminals first. Make sure they are clean and tight. Then, have the battery load tested. An old battery can cause all sorts of weirdness. Think of it like this, if your car battery doesn't function as a filter for electrical current, your ECU will suffer. Also, what kind of aftermarket electronics are installed in your car?

Thanks for the swift response. I'll check the battery terminals and do a load test. The car has an aftermarket head unit and alarm system. Could these cause the overvoltage, or are they more likely affected by it?

Okay, so the aftermarket stuff could be the problem, or at least making things worse. Think of your car's electrical system like a complex plumbing network. If someone installs the wrong kind of pipe (aftermarket part) or messes with the pressure (voltage), things can go haywire. It sounds like you are more knowledgeable than the average driver, so you'll probably be able to measure the voltage with a multimeter. That said, if you see all these weird errors again, I would still recommend taking it to a trusted workshop. It might save you a massive headache.

Thanks for the advice about parasitic draws from the aftermarket items. I took the car to a mechanic. The battery was heavily discharged and borderline defective. The repair, including a new battery and diagnostic work, cost 290€.