C8 Terminal 30 Error

That sounds like a power supply issue. I had something similar happen with my old Volvo, though it was a petrol engine. The symptoms were almost identical - struggling to start, especially in the cold. Have you checked the battery voltage? A heavily discharged or defective battery can sometimes cause that kind of problem. It could also be a short circuit somewhere, but I would start with the battery.

Thanks for the quick reply! I haven't checked the battery voltage myself, but the mechanic did say something about it being heavily discharged. Could a bad battery really cause such a specific-sounding error like "Terminal 30 continuous plus interrupted"?

Yes, a significantly discharged battery could definitely be the root cause. "Terminal 30" refers to a continuous power supply in the car's electrical system. If the battery isn't providing enough consistent power, it can trigger that kind of error. While it could still be a wiring issue or something more complex, I'd say a battery test and potential replacement is the logical first step. I'd recommend taking it to a workshop - they can properly diagnose the battery and charging system.

Thanks a lot for the advice. I took it to a workshop, and you were right! It turned out the battery was defective and causing the "Terminal 30" issue. They replaced it, and the starting problem is completely gone. The repair cost me 290€ I really appreciate your help!