C3 exhaust leak

The symptoms you describe—the check engine light, reduced engine power, and increased fuel consumption—strongly suggest a potential issue with the exhaust system. In particular, a faulty exhaust temperature sensor, as you hypothesized, could indeed be the culprit, especially if there's a leak in the exhaust system affecting the sensor's readings. I once faced a similar problem with my Citroen C3. To properly diagnose the issue, have you checked the exhaust system for any visible damage or leaks?

Thanks for the reply! I haven't checked for any leaks myself, honestly. I'm not really sure where to start looking. Any tips on where I should be looking or what to look for?

Given your intermediate level of expertise, I would strongly recommend against attempting a self-diagnosis and repair of the exhaust system. The exhaust system is a critical component for emission control, and any improper handling could exacerbate the problem or even create new ones. A leak in the exhaust system can be difficult to detect without the proper equipment and knowledge. Therefore, I suggest taking your C3 to a qualified workshop for a thorough inspection. They will be able to accurately pinpoint the source of the problem and perform the necessary repairs.

Thanks so much for the advice! I took it to a local garage and they confirmed it was a leak in the exhaust system, which caused the exhaust temperature sensor to malfunction. They replaced the faulty sensor and repaired the leak, and the total cost was around 265€. Everything seems to be running smoothly now!