RS4 NOx Sensor Issue & Check Engine Light Fix Needed

I had this exact issue on my 2020 RS5. The check engine light appeared along with exhaust issues, and diagnostic testing confirmed a faulty NOx sensor. This is a common sensor failure in these engines that triggers pollution control warnings. The problem was a damaged wiring harness connecting to the NOx sensor, not the sensor itself. The connector had corroded and lost proper contact. While initially appearing serious, the fix involved cleaning the connection points and replacing the damaged wiring section rather than the entire sensor unit. The exhaust system needs to operate with precise sensor readings to maintain proper emissions levels. When the NOx sensor loses connection or fails, the engine management system receives incorrect data about exhaust gas composition, triggering the check engine warning. Most repair shops can handle this, but it requires proper diagnostic equipment for exhaust emission testing. A certified dealer or specialized diesel engine shop will have the right tools to properly verify sensor function after repair. Before replacing the sensor completely, have them check the wiring connections first. Many times the issue stems from connection problems rather than sensor failure itself.

Thanks for sharing your experience with the wiring harness. Your reply is super helpful since I also face emission sensor issues now. The check engine light keeps coming on in my Audi RS4 and the catalytic converter seems to run inefficiently based on the air-fuel ratio readings. Did you remember roughly how much the repair cost in your case? Also wondering if youve had any recurring problems since getting it fixed?

After taking it to the mechanic, I can confirm the NOx sensor failure was resolved. The issue turned out to be exactly what I suspected, a faulty cable connection to the emission sensor. Total repair cost was 95 Euro, which is much better than replacing the entire sensor unit. The catalytic converter is now working properly since the sensor can accurately measure exhaust emissions again. The exhaust system performance returned to normal immediately after the repair, and the check engine light hasnt come back on since. Its been several months now without any recurring issues. When the wiring connection is properly fixed, these sensor failures typically dont return. The key was having a technician who knew to check the connections before jumping straight to sensor replacement. This type of repair helps maintain proper exhaust gas readings and keeps the emission control system working efficiently. Much more cost-effective than replacing components unnecessarily.

I finally visited a certified mechanic who specializes in diesel engine diagnostics. The check engine light issue in my RS4 turned out to be exactly what you described, a damaged connection in the NOx sensor wiring. The mechanic carefully tested the exhaust system and found corrosion affecting the sensor connector. Instead of replacing the entire nitrogen oxide sensor unit, they cleaned the connection points and replaced a small section of wiring. Total cost came to 120 Euro, definitely better than buying a new sensor. After the repair, my emission readings returned to normal and the check engine warning disappeared immediately. The catalytic converter efficiency improved significantly once the sensor could provide accurate readings again. Pretty relieved it was just a wiring issue rather than a major exhaust system problem. Its been running perfectly for the past two weeks with no warning lights. Really glad I got a second opinion instead of rushing to replace expensive parts unnecessarily.