Citroen C6 NOx Sensor Issue

The symptoms you describe are indeed indicative of a faulty NOx sensor. The function of the NOx sensor is to measure the levels of nitrogen oxides in your exhaust gases to ensure compliance with emissions regulations. A defective sensor can certainly trigger the check engine light and store error codes. Given the mileage, it's plausible that the sensor itself has reached the end of its service life. However, as you suggested, it is prudent to first inspect the integrity of the wiring harness and connector to the sensor. Are there any signs of physical damage, corrosion, or loose connections?

Thanks for the quick response! I'll check the wiring again. What exactly should I be looking for in terms of corrosion? Also, if the wiring seems okay, is there a way to test the sensor itself, or is it better to just replace it?

Corrosion will typically present as a greenish or whitish deposit on the metal contacts within the connector. It disrupts electrical conductivity. While it is possible to test the sensor, it requires specialized diagnostic equipment and knowledge of the expected signal outputs. Considering the relatively low cost of the sensor (approximately 95€) and the age of the vehicle, I would advise taking the car to a workshop, so you can be absolutely certain about the root cause.

Okay, I understand. Thanks for the advice! I checked the wiring as you suggested, and everything seemed fine. Given your recommendation, I took the car to a local shop. You were spot on, the NOx sensor was the culprit; they replaced it. The final repair cost was indeed 95€. The check engine light is now off. Thanks for pointing me in the right direction.