Peugeot 406 diesel engine power loss

I had similar symptoms on my 406 (diesel as well). Reduced power and an error code pointing towards the ECU. Have you tried a system reset? A defective main relay could definitely cause those issues, as it's responsible for providing power to the ECU. When the ECU doesn't get the correct power it'll throw errors or not work at all.

Thanks for the quick response! By system reset, do you mean just disconnecting the battery for a while, or is there a more specific procedure I should follow? Also, did your diagnostics tool give you any specific error codes related to the relay itself, or just general ECU malfunction?

Disconnecting the battery might help, but it might not fully reset the ECU. Some cars need a proper diagnostic tool to clear the errors and properly reset the system. In my case, the error was vague, pointing towards a general ECU fault. Given your symptoms and the error code, I'd recommend taking it to a workshop for a proper diagnosis. It's better to confirm the relay before replacing the whole ECU.

Cheers, appreciate the advice. Turns out you were spot on with the relay. Took it to a local garage, they confirmed it was faulty. Replaced it for 120€ and the car is running like a dream again. Thanks for pointing me in the right direction!