Audi S4 O2 Sensor Failure Draining Your Tank Fast

Had a similar problem with my 2010 Audi S5 last year. Have some experience working with sensor testing and basic maintenance, but left this one to professionals since it involved the exhaust system. Started with higher fuel consumption and the malfunction indicator light. Had my regular workshop run diagnostics which confirmed a faulty sensor, specifically the oxygen sensor heater circuit. The problem was actually simpler than expected: they found a damaged connection at the sensor plug. The repair involved: Full diagnostic scan, Testing the sensor circuit, Replacing the wiring connector, Clearing the check engine light, Testing emissions after repair Total cost was 95 Euro including labor and parts. Car ran much better after fixing the connection, and fuel consumption returned to normal. Before suggesting specific fixes for your S4, could you share: Current mileage on the vehicle? When was the last time oxygen sensors were replaced? Are you noticing any rough idling or power loss? Have you had any recent exhaust work done? This info would help determine if your issue matches what I experienced or might be something different.

Last service was at 109750 KM about 2 months ago. The air-fuel ratio has been off since then, and my current mileage is 111985 KM. I appreciate your detailed reply about the sensor issue. After seeing your repair process, I am noticing similar symptoms. The rough idling started right after that service interval. No recent exhaust work has been done, and as far as I know, the oxygen sensors are original. The malfunction indicator came on about a week after I first noticed the increased fuel consumption.

Thanks for providing those details, it helps confirm what I was thinking based on my previous S5 experience. Given the similar symptoms and especially with original oxygen sensors at that mileage, this is very likely the same heater element failure I encountered. While my issue was just a connection problem, in your case with original car components at 111K, a full sensor replacement might be the better route. The air-fuel ratio issues combined with rough idling strongly suggest the oxygen sensor itself is failing, not just the wiring. Key differences from my case: Your symptoms developed after service, pointing to possible disturbance of aging components, Original sensors at higher mileage than when my issue occurred, More pronounced idling problems than what I experienced Recommend getting both upstream and downstream sensors checked, as they work as a pair. My repair was cheaper because it was just wiring, but a full sensor replacement typically runs 250-350 Euro for parts and labor. Looking back, I wish I had replaced the sensor entirely when I had the wiring issue, as I ended up needing to do it about 8 months later anyway. Would save you potential future repairs to do it all at once.

Thanks for all that feedback. Your experience with the S5 really helps clarify what I should focus on with my S4. Makes perfect sense about checking both sensors since they work together. The timing of my issues starting right after the service does seem telling. Based on what you shared, I will have my mechanic check the heater element and connections first, but prepare for likely needing full sensor replacement given my mileage and original parts. The rough idle and air-fuel ratio problems match up with what you experienced too closely to ignore. Will take your advice about replacing both sensors if needed rather than trying to save money with just wiring fixes. Better to handle it all at once than deal with the malfunction indicator coming back on in a few months. Setting aside about 350 Euro for the full job seems wise based on your cost estimates.