RS4 O2 Sensor Issues, High Fuel Use & Check Engine Light

Had a similar issue with my 2021 RS5 last winter. Have some experience working on cars as a hobby mechanic. The check engine light and high fuel consumption you describe matches exactly what I experienced. The air-fuel ratio was way off due to a bad connection between the O2 sensor and its wiring harness. The automotive sensor itself was fine, but corrosion had built up in the plug. Took it to a workshop after my diagnostic tool showed the oxygen sensor circuit error code. They thoroughly checked all connections and found the damaged plug. Total repair cost was 95 Euro, mostly labor for cleaning and reseating the connection properly. Fuel consumption went back to normal right after. Before jumping to conclusions though, could you share: Which error code is stored exactly? How much has your fuel consumption increased? Does the check engine light stay on constantly or flicker? Have you noticed any rough idling or drop in power? This info would help confirm if your issue matches what I experienced or if something else might be going on.

Just had my 71071 KM service done last month. Great suggestions about checking the connections first, wish I had done that before replacing my exhaust sensor completely. In my case, the check engine light turned out to be triggered by a completely failed O2 sensor, not just a loose connection. The sensor replacement fixed both the warning light and fuel consumption issues. The workshop actually showed me the old oxygen sensor, it was totally corroded on the sensing element. If your error code specifically points to the O2 sensor circuit like mine did, might be worth having them test the sensor output voltage to be certain. My repair cost was 320 Euro including parts and labor. Has your diagnostic tool shown any specific voltage readings from the current sensor?

Thanks for the follow-up about your sensor replacement. The error code on my RS5 initially made me think I needed a full O2 sensor replacement too, but I got lucky with just the connection issue. My diagnostic tool showed fluctuating voltage readings between 0.1V and 0.9V, which actually turned out to be from the poor connection rather than a failed sensor. The normal range should be around 0.2V to 0.8V with smooth transitions. Your situation sounds more severe with the completely corroded sensing element. The workshop was right to replace the whole oxygen sensor in your case. 320 Euro is a fair price considering how crucial these automotive sensors are for proper fuel efficiency. A failed O2 sensor definitely explains your check engine light and fuel consumption issues. For others reading, if cleaning connections doesnt resolve similar symptoms, and voltage readings are erratic or nonexistent, a full sensor replacement is likely needed. Have you noticed any other improvements since the replacement besides fuel consumption returning to normal?

Thanks for the input. Yes, actually I have noticed several improvements since getting the new sensor installed! My car feels more responsive, especially during acceleration, and the engine runs much smoother at idle. Based on my fuel tracking app, I'm saving about 2L/100km compared to when the sensor was failing. The workshop mentioned during regular car maintenance that O2 sensors typically last 60,000-80,000 KM, so mine failing at 71,000 KM fits that pattern. Would recommend others experiencing similar fuel efficiency issues to check their sensor age and get proper diagnostic voltage readings first. My diagnostic tool now shows steady voltage readings between 0.2V and 0.8V, which the mechanic confirmed is exactly where it should be for optimal air-fuel ratio. Really glad I went ahead with the full exhaust sensor replacement instead of just checking connections, even though it cost more upfront. Anyone else seeing unusual fuel consumption numbers should definitely get their O2 sensors checked, it made a bigger difference than I expected.