Audi S8 NOx Sensor Issue Linked to Exhaust Leak

As someone with basic experience working on cars, I had a similar issue with my 2012 Audi S6. The check engine light came on and the diagnostic showed a NOx sensor failure. Initially, I was worried it might be the entire catalytic converter. After taking it to a workshop, they found the problem was much simpler. The cable connection to the NOx sensor had deteriorated and was causing faulty readings. The repair involved replacing the wiring harness and checking all connections in the exhaust system. Total cost was 95 Euro for parts and labor. The sensor connection issue is fairly common in these engines. In my case, fixing the wiring resolved both the check engine light and the exhaust system readings completely. To help more specifically with your S8, could you share: Have you noticed any performance issues? Are there any unusual exhaust smells? Did you get specific error codes from the diagnostic? Has the car had any previous exhaust system repairs? This would help determine if your issue matches what I experienced or if it might be something different.

Wanted to update that I got the diagnostic codes rechecked. The air-fuel ratio readings are way off and my mechanic confirmed the sensor failure is causing the check engine light. Based on inspection of the exhaust system, there is a small leak near the sensor mount. My S8 just had its service at 63885 KM and they did a full pollution control check which showed everything else working properly. No previous exhaust work has been done and the car runs smoothly otherwise, just with a slightly rough idle. Will probably need the sensor replaced, but concerned about potential damage to surrounding components from the leak. The codes point specifically to the post-cat sensor. Looking at repair quotes ranging from 350 Euro to 500 Euro for parts and labor. Would appreciate hearing if anyone else faced similar repair costs.

Thanks for the update on your S8. When my check engine light issue occurred, I also had those irregular air-fuel readings. The exhaust leak near the sensor mount is actually quite common with these models and often develops right where the emission sensor threads into the exhaust system. The rough idle you mentioned matches exactly what I experienced, it was a telling sign of the NOx sensor not reading correctly. The quotes you got for 350-500 Euro are in line with what most shops charge for this repair on our cars. The sensor itself usually runs about 280 Euro, plus labor for accessing the exhaust system and ensuring proper calibration. One important note from my experience: make sure they check the mounting threads in the exhaust system where the sensor sits. My leak was partly due to damaged threads, which required a minor repair to ensure the new sensor sealed properly. This prevented future leaks and sensor failures. The good news is since your recent pollution control check showed everything else working fine, its unlikely the catalytic converter is affected. The quick resolution of replacing just the faulty sensor should get your S8 running smoothly again.

Thanks for the input. After getting multiple opinions, I decided to move forward with the sensor replacement. The repair cost came to 425 Euro total, right in the middle of the quoted range. The mechanic found worn threading where the NOx sensor mounts, just like others mentioned. They cleaned and repaired the threads before installing the new sensor. The check engine light is now off and the air-fuel ratio readings are back to normal. The exhaust leak is also fixed and the rough idle is gone. Worth noting for others with similar issues, having the full pollution control check done recently helped confirm this was just a sensor failure and not a larger catalytic converter problem. The diagnostic codes pointing to the post-cat sensor were accurate in identifying the exact issue. Going to monitor the exhaust system for a few weeks to ensure everything stays properly sealed around the new sensor mount. So far, the repair seems to have completely resolved both the sensor failure and associated symptoms.