Peugeot 307 reduced engine power

Reduced power and an error definitely points to ECU issues. Have you tried a system reset to see if that clears the error? Also, have you had the car diagnostics run to pull the specific error code? That would help narrow down the problem. Intermittent issues with the main relay can sometimes cause these symptoms too.

Thanks for the quick reply! System reset - you mean just disconnecting the battery for a bit? And yeah, I pulled the error code - nothing specific. Could a faulty main relay actually cause the ECU to throw a generic error?

Disconnecting the battery might help, but a proper diagnostic tool is better for a full system reset. Yes, a failing main relay can absolutely cause the ECU to behave erratically and throw generic errors due to inconsistent power supply. Given the symptoms and mileage, it's worth checking the relay before assuming the ECU itself is toast. Still, with those vague error codes, I'd advise taking it to a workshop.

Thanks, I appreciate the advice. It was indeed the main relay. Replaced it and all errors are gone. Cost me 120€ in the end.