Xsara Oxygen Sensor Fault

Ah, the dreaded Xsara O2 sensor! Yes, I have wrestled with this beast myself on several occasions with older PSA vehicles. The symptoms you describe are textbook. Considering the age, the sensor is likely past its prime. Before throwing parts at it, have you checked the voltage at the sensor connector with the engine running? A faulty heater circuit within the sensor can also trigger those symptoms, even if the sensor itself is still sort of functional. This would be an easier fix than replacing the whole sensor, though access can be a pain!

Thanks! I haven't checked the voltage at the sensor connector. Can you tell me how to do that? Which pins should I test, and what voltage range should I look for? Any precautions to avoid damaging anything?

Given that you are encountering elevated fuel usage on top of the error message, I would hazard a guess the O2 sensor itself is causing the issue, not merely the heater circuit. Considering the age and mileage and your description of a potentially defective plug connection I'd simply visit your local repair workshop and have them inspect the sensor. If they determine, as I'm sure they will, that it must be replaced you can expect the repair to cost you around 95€. It might cost a bit more time and money in the short-term, but you save time on potentially fruitless repairs.

Thanks, I took it to my shop. They said the oxygen sensor was faulty, probably the connection like I thought. They replaced it for about the price you said. The check engine light is off, the error is gone, and the fuel consumption is back to normal. Thanks for the help!