GT86 Undervoltage Error

I had a similar issue with my 2016 BMW 3 series a while back. What is the measured voltage across the battery terminals when the engine is off, and then again when the engine is running? Also, have you had the battery's health checked recently? A heavily discharged or defective battery can certainly cause these symptoms. An alternator issue could also be a culprit, but let's rule out the simple stuff first.

I haven't measured the voltage, but is it possible that a bad battery is causing all these issues? I thought a bad alternator was more likely.

It is indeed possible. While an alternator issue could definitely be at fault, a failing battery can cause a cascade of electrical problems in modern vehicles. Given the symptoms you're describing, I would not exclude the battery from the list of potential root causes. However, without proper diagnostic tools and measurements, it is hard to assess the condition of the battery. Therefore, I would suggest that you consult a qualified technician to examine the car's electrical system.

Thanks for the advice. It turned out the battery was defective. Replaced it for 290€ and it fixed everything.