Audi Q7 NOx Sensor Failure Causing Exhaust Leak

Had the same issue on my 2011 Audi Q5 TDI. As someone with moderate car repair experience, I understand the concern when the check engine light comes on, especially with diesel engine problems. The emission sensor issue is quite common in these models. Initially thought it was a major exhaust system problem, but it turned out to be a faulty connection to the nitrogen oxide sensor. The workshop found a damaged cable harness connecting to the NOx sensor, which was causing incorrect readings and triggering the check engine warning. Total repair cost was 95 Euro for diagnostics and fixing the connection. Much better than replacing the entire sensor unit. The fix took about an hour and resolved both the warning light and exhaust issues. Few questions to better assist: Have you scanned for specific error codes? Is there any noticeable performance loss? Does the check engine light stay constant or flash? Have you noticed any unusual exhaust smoke? These details would help determine if your issue matches what I experienced or if its something different in the exhaust system.

Thanks for your feedback. Last service was 2000km ago and current mileage shows 74540km. The check engine light remains constant, and I pulled error code P0101. No performance issues noticed yet, but there is black smoke occasionally during acceleration. Running a complete air-fuel ratio test revealed inconsistent readings. The pollution control system seems affected since the exhaust emissions are higher than normal. Looking to get this resolved before it leads to more serious problems with the emission sensor system.

Thanks for providing those details. The black smoke during acceleration combined with the P0101 code points to a different issue than what I initially experienced with my Q5. While my problem was a simple wiring issue, your symptoms suggest a mass airflow (MAF) sensor problem affecting the air-fuel ratio. The exhaust smoke and inconsistent readings indicate the diesel engine is running rich, meaning too much fuel compared to air. This directly impacts the pollution control system and can trigger false NOx sensor readings. In my case, after fixing the initial sensor connection, I also had to deal with a similar MAF issue about 6 months later. The repair involved: Cleaning the MAF sensor, Checking air intake for restrictions, Testing the exhaust system for proper flow The complete fix cost 320 Euro, including parts and labor. Would definitely recommend getting this checked soon, as running rich can damage other emission sensor components and lead to more extensive exhaust system repairs. Based on your mileage and symptoms, cleaning the MAF sensor might solve the issue. If not, replacement would be the next step. Have you checked if the air filter is clean? A clogged filter often contributes to these symptoms.

Thanks for the input. After getting your feedback, I checked the air filter and it was pretty dirty, definitely overdue for replacement. Had it professionally diagnosed yesterday, and as suggested, the mass airflow sensor was the main culprit affecting the air-fuel ratio. The technician confirmed the black smoke was due to the engine running rich, which explains the pollution control system warnings. They cleaned the MAF sensor and replaced the air filter. The check engine light is now off and the exhaust emissions have normalized. Total repair came to 280 Euro including diagnostics, parts, and labor. Much better than potentially damaging the catalytic converter or other components by letting it run too long. No more black smoke during acceleration and the nitrogen oxide readings are back to normal ranges. Will definitely keep a closer eye on the air filter maintenance schedule going forward. Thanks again for helping point me in the right direction for diagnosis and repair.