Audi Q8 NOx Sensor Failure causing Exhaust Issues

I had a pretty similar issue with my 2016 Audi Q7 last year, and I have some experience working on basic car maintenance. The symptoms you describe match what I encountered, check engine light and exhaust issues. In my case, the NOx sensor was giving false readings due to a loose wiring connection. The sensor itself was actually fine, but the cable connection had become corroded and loose over time, affecting the air-fuel ratio readings. The fix was relatively straightforward. The workshop found the faulty connection during diagnostics, cleaned the contacts, and properly secured the wiring harness. Total cost was 95 Euro including diagnostic time. Since the repair, my exhaust system has been working perfectly with no error codes. Before jumping to conclusions about sensor failure, it would help to know: Have you scanned for specific error codes? Is there any noticeable drop in performance? Does the check engine light stay on constantly or flicker? Can you describe the exact location of the exhaust leak? With this information, I might be able to confirm if your issue matches what I experienced with my vehicle. The catalytic converter and exhaust system components are quite similar between our models.

Last service was done at 121175KM. I have actually gone ahead and scanned for the codes myself which returned P0420 and P0430, suggesting potential issues with the catalytic system efficiency. The check engine light has been steady since it first appeared, and there is a noticeable performance drop especially during acceleration. The emission sensor readings are inconsistent, which could point to either sensor failure or deeper pollution control system problems. The exhaust fumes seem to be coming from the area near the catalytic converter. I noticed the air-fuel ratio seems off as well, as the engine runs slightly rough at idle. Given these symptoms, I am leaning toward a more serious issue than just a loose connection, but wanted to check if anyone has experience with similar combinations of problems before visiting a repair shop.

Thanks for the additional details. Those codes youre seeing match a more complex issue than what I initially encountered with my Q7. While my problem was solved with a simple wiring fix, your symptoms point to potential catalytic converter degradation. After you mentioned the rough idle and the specific location of the exhaust leak, it reminds me that I actually faced a similar combination of issues about 6 months after my initial NOx sensor problem. The air-fuel ratio was noticeably off, and the pollution control system wasnt functioning correctly. In my case, the catalytic converter had partially failed internally, causing both the performance issues and triggering those same error codes. The physical damage to the catalytic converter was also causing the exhaust leak you described. The repair ended up being more extensive than my previous fix. The total cost was around 1200 Euro for a new catalytic converter plus labor. While expensive, it resolved all issues, the check engine light cleared, performance returned to normal, and the exhaust leak stopped. Based on your codes and symptoms matching my experience exactly, I would prepare for potential catalytic converter replacement rather than just sensor failure.

Thanks for sharing your repair experience. Those error codes P0420 and P0430 definitely make me think this is more serious than I initially suspected. The steady check engine light combined with the performance issues and rough idle do suggest a catalytic converter problem rather than just a sensor issue. I have noticed the problem is getting worse over time. The diesel engine seems to struggle more during cold starts, and the emission sensor readings continue to be erratic. The exhaust leak near the catalytic converter area has also become more noticeable, especially on cold mornings. Given all the symptoms and comparing them to your experience, I think I will need to prepare for a potential catalytic converter replacement. While 1200 Euro is quite steep, it seems worth it to restore proper engine performance and fix the exhaust system issues properly. I will schedule an appointment with my regular shop this week to have them confirm the diagnosis and get a detailed quote for the repair. The declining performance and increasing exhaust problems make me want to address this sooner rather than later to prevent any additional damage to the pollution control system. Thanks again for sharing your similar experience, it helped confirm my suspicions about the root cause.