Renault Spider ECU defective

The symptoms you describe, namely reduced engine power coupled with an ECU fault code, are indicative of a potential issue with the engine control unit. Have you observed any other unusual behaviors, such as difficulty starting the engine, stalling, or erratic idling? Could you provide the specific error code retrieved from the error memory? This information would be beneficial in narrowing down the possible causes.

Starting is fine, and idle is normal. The exact error code is not at hand right now. But I read somewhere that a defective main relay could cause similar issues. Could that be the problem?

A defective main relay is indeed a plausible cause for the symptoms you are experiencing. The main relay provides power to the ECU, and if it is failing intermittently, it could lead to the ECU malfunctioning and triggering the error code. Given your expertise is limited, I would advise against attempting repairs yourself. The vehicle should be inspected by a qualified technician who can diagnose the issue accurately and perform the necessary repairs.

Thanks a lot for the info! Took it to a workshop, and you were spot on. It was the main relay. They replaced it, cleared the error memory, and everything's running smoothly again. Cost me 120€, which is fine. Appreciate the help!