Citroen C3 Reduced Engine Power After Tuning

That's a common issue with tuned engines. What diagnostic code are you getting? Have you used an auto scanner to check for other fault codes besides the one indicating reduced power? Sometimes multiple codes can point to the same underlying problem, or narrow down the possibilities.

The code is Pxxxx, related to fuel pressure. Could this error be triggered by the faulty chip tuning?

Pxxxx and fuel pressure issues after tuning definitely point towards that as the likely culprit. The aftermarket tuning likely altered the fuel maps incorrectly. You could try reverting to the stock ECU map. However, to prevent further problems, it's best to have it professionally checked. The cost for diagnosis and correction shouldn't be too high.

Thank you for confirming my suspicions. I had the car inspected and the third-party chip tuning was indeed faulty, causing the engine issue. The repair, involving reverting to the stock ECU map and clearing the error codes, cost 180€.