Seat Exeo reduced engine power issue

Sounds like a similar issue I had with my old Opel. Reduced engine power and a stored error... could be a faulty sensor. Have you checked the usual suspects like the mass airflow sensor or the oxygen sensors? What error code are you getting?

I haven't checked those yet, but a mechanic told me it could be the engine control unit itself. Could a defective main relay really cause those symptoms? I mean, really?

Absolutely! A faulty main relay can definitely cause those symptoms. It happened to my Opel. The relay provides power to the engine control unit. If it's failing, it can cause intermittent power loss, leading to reduced engine performance and error codes. I would suggest checking the relay before condemning the whole engine control unit. If the relay is the issue, it's a relatively cheap fix. Otherwise you need to visit a workshop.

Big thanks! It was indeed the main relay, just like you suspected. Replaced it, and everything is running smoothly again. Cost me around 120€. I was pulling my hair out trying to figure out what was wrong, so thank you for your help! You saved me a lot of time and frustration.