Audi S6 NOx Sensor vs Catalytic Converter Mystery

Had a similar situation with my 2013 Audi S4 3.0T. The emission sensor failure symptoms matched exactly what you describe, check engine light and visible exhaust leaks. The nitrogen oxide sensor was actually fine, but the wiring harness connecting to it had deteriorated. The exhaust system was letting fumes escape because the connection was loose, triggering the pollution control system warning. This turned into a more serious repair since the faulty wiring had caused the catalytic converter to work harder than necessary. The entire connection assembly needed replacement along with some exhaust components. Key symptoms that confirmed it wasnt just a sensor: Visible exhaust fumes around connection points, Engine running slightly rough at idle, Check engine light staying on after clearing codes The repair involved: New wiring harness, Connector assembly replacement, Exhaust system inspection, Full diagnostic system reset For your S6, recommend getting the wiring connections checked before replacing any sensors. The 4.2L engine is known for similar issues around the 120000km mark. A proper diagnosis will show if its just a loose connection or if theres actual component failure.

Thanks for sharing your experience. My issue turned out very similar, especially the part about the wiring harness. The check engine light symptom and exhaust leak match exactly what I went through with my S6. Quick question, do you remember roughly how much you ended up paying for the full repair? Also wondering if you had any other emission sensor or exhaust system problems since getting it fixed? The reason I ask is my repair was done about 8 months ago, and I just noticed some odd engine behavior starting again. Want to know if I should expect more issues with the pollution control system.

Glad I could help with that initial advice. The NOx sensor failure on my S6 was definitely resolved after fixing the cable connection issue, it cost 95 Euro for the repair. Haven't experienced any emission sensor problems since then. The exhaust system has been running perfectly after the wiring fix, with no check engine lights or pollution control warnings. If youre seeing odd engine behavior again, it might be unrelated to the previous sensor issue. Eight months is a long time for the same problem to reappear if it was properly fixed. The pollution control system on these engines is pretty reliable once the wiring connections are properly secured. Sounds like you might want to get a diagnostic scan to check if its a different sensor acting up this time.

Thanks for the input. Just got back from the mechanic and wanted to share how it turned out. The air-fuel ratio was way off because of a failing sensor connection, just like you mentioned. My mechanic found the exact same wiring issue. Total repair came to 120 Euro which included: New wiring harness installation, System diagnostic, Labor costs The diesel engine is running smooth now with no check engine light. They also did a full check of the catalytic converter while they were at it, fortunately it was still in good shape despite the sensor problems. Really glad I asked here first. Saved me from potentially getting overcharged or having unnecessary parts replaced. Will keep an eye on the pollution control system but so far everything seems fixed.