Failing O2 Sensor Spikes Audi's Fuel Consumption

I have some experience with similar issues on my 2005 Audi A4 2.0T. The malfunction indicator light was also the first sign, followed by increased fuel consumption. After checking the air-fuel ratio readings, my mechanic confirmed it was indeed related to the oxygen sensor. The main issue turned out to be a faulty connection between the oxygen sensor and its plug. The heater element was actually fine, but the wiring had deteriorated. The complete diagnostic and repair took about 2 hours, costing 95 Euro including labor. Before jumping to conclusions, could you share: Have you noticed any rough idling? Did you get specific error codes from the diagnostic? Are there any unusual exhaust smells? Has the car had any recent exhaust work done? The check engine light can be triggered by various sensor-related issues, so these details would help narrow down if your problem matches what I experienced. Getting the right diagnosis is crucial since replacing parts unnecessarily can be expensive.

Latest service was at 81550 KM, thanks for the detailed response. My car also shows rough idling especially when cold, and I got the P0135 error code from the diagnostic. No unusual exhaust smells detected, and the exhaust system is all original. Based on this input, it sounds like sensor testing would be the logical next step. At least the repair time and cost you mentioned seems reasonable if it turns out to be a similar issue.

Thanks for providing those additional details, sounds very familiar to what I experienced. The P0135 code specifically indicates an O2 sensor heater circuit malfunction, which aligns perfectly with the symptoms you described. When I had this issue, the rough cold idle was exactly the same. The fact that your exhaust system is all original and shows no unusual smells is actually good news, it suggests the oxygen sensor itself is the likely culprit rather than a larger exhaust system problem. In my case, having the original exhaust components helped rule out any installation-related issues that sometimes occur with aftermarket parts. I highly recommend getting the sensor testing done to verify the diagnosis. When they tested mine, they specifically checked the heater circuit resistance values which confirmed the faulty sensor. The repair was straightforward, my mechanic had the oxygen sensor replaced in under 2 hours and the total cost came to around 120 Euro including parts and labor. Just keep in mind that continuing to drive with a faulty sensor can affect your vehicle emissions and potentially cause additional fuel system issues. Getting it fixed sooner rather than later will help prevent any secondary problems.

Thank you for clarifying those symptoms, I will definitely proceed with the sensor testing. The P0135 error code matching your experience gives me more confidence about the diagnosis. Having the original exhaust system does seem to help narrow down the issue to just the oxygen sensor and its heater element. My mechanic can get me in next week for testing. The rough idle during cold starts was making me nervous about potential bigger problems, but the malfunction indicator showing the specific O2 sensor heater circuit code is reassuring. If the repair time and costs end up similar to what you experienced, that would be very manageable. I plan to get it fixed promptly since I want to avoid any further vehicle emissions issues or additional fuel system damage. Will update once the sensor testing is complete to confirm if replacing the oxygen sensor resolves these symptoms.