Renault Modus reduced engine power after chip tuning

Reduced engine power and a stored error code could definitely be a sign of trouble after chip tuning. The tuning process alters the car computer's (ECU) parameters, and if it's not done correctly, it can lead to issues. The 'check engine' dashboard light should also be on. Have you checked the diagnostic code with a scan tool?

Yeah, the check engine light is on. I did check the diagnostic code, it showed something about fuel trim being off. But what exactly could be wrong with the chip tuning itself? Is it a software thing?

The chip tuning likely overwrote the factory settings in the ECU, which governs fuel injection, ignition timing, and other critical parameters. If the new settings are outside the safe operating range for your engine, or are incompatible with other components, it throws the fuel trim off. The workshop can either revert to the original settings or adjust the chip tuning, but the error suggests the latter is too risky. I would recommend taking it to a trusted shop to avoid major damages.

Thanks for the info. I took it to a shop, and they confirmed the chip tuning was the issue. They reverted it back to the original settings and cleared the error. Cost me 180€, but at least it's running normally again.