C4 Diesel Exhaust Leak

The symptoms you describe – check engine light, reduced power, increased fuel consumption – are classic indicators of a fault within the emission control system. Given that you suspect the exhaust, it's plausible that the exhaust temperature sensor is faulty due to a leak in the exhaust system itself. A leak would alter the temperature readings the sensor sends to the engine control unit (ECU), triggering these symptoms. Do you observe any unusual sounds or smells, or any visible signs of exhaust leakage?

I haven't noticed any unusual sounds or smells, but now that you mention it, I think I might have seen some condensation around one of the joints in the exhaust pipe when the engine is cold. Could a small leak really cause all these problems?

A seemingly minor exhaust leak can indeed precipitate significant operational issues. The exhaust temperature sensor is critical for proper emission control and engine management. The ECU relies on accurate temperature data to optimize fuel injection and timing. Erroneous readings due to an exhaust leak can disrupt this delicate balance, leading to reduced power and increased fuel consumption. Given your observation of condensation, I would advise a thorough inspection by a qualified technician. Exhaust leaks release gases before they can be properly treated by the catalytic converter, which leads to sensor failure. It is very likely that you will need to visit a workshop.

I took it to the shop, and you were spot on. It was a faulty exhaust temperature sensor caused by a small leak in the exhaust. Sensor replacement and leak fixed, total cost 265€. Runs like new again!