C1 reduced engine power

Sounds familiar. I had similar symptoms with my C1. Did you check the main relay? A defective relay can cause ECU problems.

Thanks for the tip! Where exactly is the main relay located, and how can I check if it's defective?

It's usually near the ECU. Honestly, it's best to have a workshop diagnose it. They can check the relay and the ECU properly. I ended up needing a new relay.

Thanks a lot for your help! I took it to a workshop, and you were right – the main relay was defective. They replaced it, and it cost me 120€. Now the C1 runs perfectly again!