Audi A4 O2 Sensor Issues Drive Up Fuel Consumption

I had a comparable issue with my 2014 Audi A5 2.0 TFSI last year, and having some experience working on cars, I initially thought it was the exhaust sensor too. The symptoms were very similar, check engine light and increased fuel consumption. After taking it to the workshop, they found that the issue was actually a loose connection in the wiring harness connected to the air-fuel ratio sensor. The connection had corroded slightly, causing intermittent signal issues. The repair was relatively straightforward, they cleaned the connector and replaced the damaged cable section, costing 95 Euro including labor. The catalytic converter was also inspected during diagnostics since these symptoms can sometimes indicate catalyst issues, but it was functioning normally. Before suggesting more specific advice for your case, could you share: Current mileage on your A4, When was the last time you had a full service, Are there any other unusual sounds or behavior, Does the check engine light flash or stay constant, Have you noticed any specific driving conditions when the problem occurs This information would help determine if your issue is similar to what I experienced or if it might be something else.

Thanks for the detailed explanation. My A4 has 106023 KM and had its last service 3 months ago. The check engine light stays constant rather than flashing. There are no unusual sounds, but I notice the issue is more pronounced during cold starts. The O2 sensor issue seems plausible given the symptoms, though your experience with the wiring harness is interesting. I will definitely have the workshop check the wiring connections along with the automotive sensor itself. The steady check engine light at least suggests it is not a critical misfire affecting the catalytic converter. Before booking the repair, I will gather the exact error codes to share with the workshop. This should help narrow down whether we are dealing with an exhaust system issue or an electrical problem like in your case.

Thanks for providing those details. After my wiring harness experience, I had similar cold start issues appear again on my A5 about 6 months later. This time it actually was the oxygen sensor that needed replacement. The symptoms during cold starts are particularly telling, this matches exactly what happened with my car when the O2 sensor finally failed. The engine management system relies heavily on the oxygen sensor readings to adjust the air-fuel mixture, especially during warm-up. In my case, the replacement sensor cost 180 Euro plus 120 Euro labor. The workshop also performed a diagnostic check of the entire exhaust system while they were at it, which was worthwhile for peace of mind. They confirmed the catalytic converter was still in good condition, which was a relief since that would have been a much more expensive repair. Since your car has similar mileage to what mine had, and the symptoms match almost exactly, I would bet you are dealing with a genuine sensor issue rather than wiring. The fact that it started showing up after your recent service might be coincidental, these sensors typically start failing around the 100000 KM mark in our engines. Getting those error codes is definitely the right next step. When you do, feel free to share them here for more specific guidance based on what I learned through my repair experience.

Thanks for the helpful information about your A5 repair experience. My A4 has 106023 KM and had its last service 3 months ago. The check engine light stays constant rather than flashing. There are no unusual sounds, but I notice the issue is more pronounced during cold starts. The O2 sensor issue seems plausible given the symptoms, though your experience with the wiring harness is interesting. I will definitely have the workshop check the wiring connections along with the automotive sensor itself. The steady check engine light at least suggests it is not a critical misfire affecting the catalytic converter. Before booking the repair, I will gather the exact error codes to share with the workshop. This should help narrow down whether we are dealing with an exhaust system issue or an electrical problem like in your case.