Failing O2 Sensor Drains Audi S8 Fuel Economy

I dealt with a very similar issue on my Audi A8 4.2L from 2 years newer. As someone with moderate experience working on Audis, the symptoms you describe match exactly what I encountered, poor fuel economy and check engine light triggered by the oxygen sensor heater element failure. In my case, after sensor testing revealed the malfunction, my mechanic found that the wiring connection to the O2 sensor was corroded, affecting the air-fuel ratio readings. The fix involved cleaning the connector and replacing one damaged wire, rather than the entire sensor unit. Total repair came to 95 Euro and took about 2 hours of labor. The vehicle emissions and fuel economy returned to normal immediately after the repair. The check engine light cleared after a few drive cycles. Could you share: Your vehicle year, Which O2 sensor is showing the fault (bank 1 or 2, upstream or downstream), Any other diagnostic codes present, Whether you notice any rough idle or hesitation This information would help determine if your issue matches what I experienced or might need different attention.

Last service performed at 128500KM about 3 months ago. Thanks for sharing your experience. My diagnostic shows Bank 1 Sensor 1 (upstream) fault code. No other codes present and no rough idle issues noted. The exhaust system and all other car components appear to function normally. The air-fuel ratio inconsistency and increased consumption started about 2 weeks ago. I will have the connections checked first since your repair solution sounds promising and cost-effective. Good to know a complete sensor replacement might not be necessary if the wiring is the root cause. Did you notice the heater element issue caused any long-term impact on your catalytic converter performance after fixing the wiring?

Thanks for those details. Since you mentioned the fault is on Bank 1 Sensor 1, its exactly the same position where my faulty sensor issue occurred. The good news is I experienced no long-term effects on the catalytic converter after fixing the wiring. The malfunction indicator light stayed off and vehicle emissions remained stable after the repair. I monitored the air-fuel ratio readings for several months just to be sure, and everything performed normally. One important detail from my experience, when the mechanic inspected the wiring, they found moisture had gotten into the connector housing, accelerating the corrosion. They applied dielectric grease during reassembly to prevent future issues. In 18 months since the repair, I have had zero related problems. Given your car shows similar symptoms and no additional fault codes, theres a good chance youre dealing with the same wiring issue rather than a complete sensor failure. The check engine light should clear once the proper electrical connection is restored. Just make sure whoever checks it tests the sensor heater circuit completely before and after the repair to confirm the fix worked.

Going to have my mechanic look at this tomorrow. Your insight on the connector moisture issue is particularly helpful since we have had a lot of rain lately. Makes sense this could be the root cause, and I like that the fix doesnt require replacing the entire oxygen sensor assembly. The timing of your catalyst converter feedback is perfect too, I was concerned about potential damage from running rich these past weeks. Good to hear no long-term exhaust system issues developed in your case. Will definitely request thorough sensor testing and have them apply dielectric grease during reassembly. 95 Euro for the repair sounds much better than the 400 Euro+ quote I got for a complete sensor replacement. Will update the forum once I get the results from the wiring inspection and hopefully confirm another successful connector repair case for future reference.