Q8 Check Engine Light Points to Faulty O2 Sensor

As someone with some automotive repair experience, I encountered a very similar issue with my 2017 Audi Q7. The symptoms were identical, check engine light and poor fuel efficiency. The O2 sensor was indeed the culprit, specifically a faulty connection in the wiring harness to the automotive sensor. The workshop first performed a complete diagnostic scan which pointed to the air-fuel ratio being off. They traced it to a damaged cable connection at the exhaust sensor. The fix was straightforward, they replaced the wiring connector and properly secured it. Total cost was 95 Euro and the repair took about an hour. Since your symptoms match what I experienced, it could be the same issue. However, to provide more specific guidance, could you share: 1. What error code is showing up in your diagnostics? 2. Have you noticed any other symptoms besides the check engine light and reduced fuel efficiency? 3. Has there been any recent work done on the exhaust system? 4. When was the last time the O2 sensor was replaced? These details would help determine if your issue is similar to what I encountered or if it might be something else in the system.

I just had my latest service at 72000 KM and the issue started about a month after. The diagnostic scan showed error code P0131, indicating a low voltage from the O2 sensor. No other work has been done on the exhaust system, and this would be the first O2 sensor replacement since the car was new. I also noticed a slight hesitation during acceleration that wasnt there before, along with the poor fuel economy and check engine light issues already mentioned. Looking at the service records, the O2 sensor has never been replaced, so maybe its just reached the end of its life. I will schedule a proper diagnostic check to confirm before replacing any parts.

Based on the P0131 code and your symptoms, this definitely sounds like a failing oxygen sensor. I had similar symptoms in my Q7 before replacing the sensor, including that hesitation during acceleration you mentioned. In my case, the oxygen sensor lasted about 80000 KM before needing replacement. Car maintenance records show oxygen sensors typically need replacement between 60000-80000 KM, so your timing fits the typical wear pattern. When my catalytic converter was inspected during the sensor replacement, the technician confirmed the early sensor replacement actually helped prevent potential damage to the catalytic converter, which would have been much more expensive to fix. Since you mentioned having the initial symptoms right after a service, its worth having them check if any connections to the exhaust sensor might have been disturbed during that work. However, given the error code and mileage, a failing sensor is the most likely cause. The complete O2 sensor replacement on my Q7 cost around 350 Euro including labor. Make sure they check both upstream and downstream sensors while they have it up on the lift, as they often wear at similar rates. Have you noticed if the hesitation is more pronounced when the engine is cold or at operating temperature?

Thanks for all the detailed feedback. The hesitation is definitely more noticeable when the engine is cold, especially during the first 10-15 minutes of driving. It smooths out somewhat once everything warms up, but the poor fuel efficiency remains constant. I called the workshop and scheduled a diagnostic appointment for next week. I will have them inspect both O2 sensors as suggested, since the cost difference to check both while the car is already lifted isnt much. The 350 Euro estimate for the sensor replacement seems reasonable compared to other repairs, especially if it prevents damage to other exhaust system components. The service record check was helpful, knowing these typically need replacement between 60000-80000 KM makes me feel better about the timing. Will make sure to ask them to inspect all the exhaust sensor connections too, just to rule out any wiring issues from the recent service.