Peugeot 106 ECU Fault

Reduced power and a stored error? Sounds like ECU trouble. Have you checked the wiring and connections to the ECU? Sometimes a loose connection or corroded terminal can cause similar problems. Is the engine light on constantly, or is it intermittent? Rule out the simple things first before condemning the computer module.

I've looked at the wiring and connections, but haven't tested them. The engine warning light is always on. If the main relay has failed, does that mean the ECU needs replacing?

If the light's constantly on and you've checked the easy stuff, and you're thinking it's the main relay affecting the ECU, it's getting serious. A faulty relay could mess with the ECU's performance, but it's not guaranteed. It's like saying a headache is always a brain tumor. Still, with those symptoms and mileage, you are likely on the right track. Get it properly diagnosed at a workshop before replacing parts. Automotive electronics can be tricky.

I consulted a workshop. The diagnostics confirmed a defective main relay, resulting in ECU malfunction. The repair cost 120€. Thanks for the help.