Audi A4 NOx Sensor Issues: Exhaust and Error Codes Alert

As someone with some car repair experience, I dealt with a very similar issue on my 2019 Audi A3. The check engine light came on, and the exhaust system was showing similar symptoms to what you describe. The diagnostic scan revealed problems with the nitrogen oxide sensor signals. After inspection, the workshop found that the issue was actually a damaged cable connection to the NOx sensor, rather than a complete sensor failure. The air-fuel ratio readings were all over the place because of this faulty connection. The fix involved replacing the wiring harness and checking all connections to ensure proper communication between the sensor and the engine control unit. Total cost was 95 Euro for parts and labor, which was much better than replacing the entire sensor. Before suggesting more specific solutions, could you share: Current mileage on your A4, Whether the check engine light is constant or flashing, Any changes in fuel consumption, Specific error codes from the diagnostic scan This information would help determine if your issue matches what I experienced and could save you from unnecessary parts replacement.

My A4 has 110749 KM and the last service was done just 2 months ago. The emission sensor issues you mentioned sound very similar to what I am experiencing. The check engine light stays constant and I noticed about 15% higher fuel consumption lately. The error codes from my scan are P0420 and P0133, which point to catalyst system efficiency and oxygen sensor circuit issues. I am hoping it is just a wiring issue like in your case, as replacing the entire exhaust system would be quite expensive. The nitrogen oxide readings are indeed inconsistent according to the diagnostic report from my scan tool. Could the pollution control system be affected by this as well?

Thanks for providing those details. Your situation sounds even more similar to what I experienced with my A3 now that I see the error codes and symptoms. The increased fuel consumption of 15% matches exactly what I noticed before fixing the wiring issue. The P0133 code on my car was also related to the wiring problem affecting the oxygen sensor circuit. The pollution control system was definitely impacted because the engine management system wasnt receiving accurate data from the NOx sensor to properly adjust the air-fuel mixture. In my case, the catalyst system efficiency code (P0420) cleared up after fixing the wiring and sensor connections. The inconsistent nitrogen oxide readings were caused by the same electrical issue, not a failed catalyst. Before replacing any major components, have a technician check: 1. All sensor wiring connections for corrosion or damage 2. Ground connections near the exhaust system 3. The connector pins on both the NOx and oxygen sensors The total repair should be significantly less than a catalyst replacement if its just an electrical issue like mine was. The emission sensor system usually returns to normal function once the connections are restored. A quick voltage test at the sensor connectors would confirm if your issue matches my experience with the wiring harness problem.

Thanks for sharing those specific error codes and mileage details. The constant check engine light and 15% increase in fuel consumption definitely align with what others experienced with similar wiring issues. Having P0420 and P0133 codes together strongly suggests an electrical problem affecting the catalytic converter readings through faulty sensor data, rather than an actual catalytic converter failure. Given that my last service was only 2 months ago and these issues started recently, a wiring problem seems the most likely cause. The inconsistent nitrogen oxide readings from my scan tool match what you described with your sensor failure experience. Its good to know the pollution control system could be giving false readings due to bad sensor data rather than actual system problems. I will have my mechanic check the wiring connections and ground points before considering any expensive exhaust system replacements. A voltage test of the sensor connections makes sense as a next diagnostic step. Hopefully my repair costs will be closer to the 95 Euro range you mentioned rather than the much higher cost of replacing major components. Will update once I get the wiring connections checked out.