RS5 O2 Sensor Causing Low MPG and CEL Issues

Had the same O2 sensor problem on my 2010 S5. The check engine light and poor fuel economy are classic symptoms of a failing oxygen sensor, which is a common car maintenance issue after several years of use. The exhaust system relies heavily on proper O2 sensor readings to maintain optimal fuel mixture. When the sensor starts failing, the engine management system defaults to a rich fuel mixture as a safety measure. This explains the increased fuel consumption you noticed. My repair involved replacing both upstream and downstream O2 sensors since they work as a pair in the exhaust sensor system. The fix restored normal fuel efficiency and cleared the check engine light immediately. The job took about 2 hours at the shop. Not a major repair in terms of complexity, but definitely something that needs attention soon. Continuing to drive with faulty O2 sensors can lead to catalytic converter damage, which is much more expensive to fix. For accuracy, get the specific sensor codes read. Sometimes what appears to be an O2 sensor issue could be related to other components in the exhaust system.

Thanks for sharing your experience. I faced a similar air-fuel ratio issue on my RS5 last month. The check engine light came on and the mechanic confirmed it was indeed the O2 sensor. They fixed it quickly and the car runs perfectly now. What did you end up paying for the repair? And has everything been running smoothly since the sensor replacement? Just wondering if any other exhaust system issues popped up afterward.

Thanks for asking about the follow-up. After taking my S5 to the mechanic, they discovered the issue was actually a loose connection at the oxygen sensor plug, not a completely failed sensor. A thorough car maintenance check revealed the wiring needed securing and proper cleaning of the connections. The repair cost came to 95 Euro, which was much better than replacing the entire automotive sensor unit. The check engine light cleared immediately after they fixed the cable connection. The fuel economy returned to normal within a few tank fills. Since this repair 6 months ago, the exhaust system has been working flawlessly. No additional O2 sensor issues or other exhaust-related problems have emerged. The initial diagnosis of a completely failed sensor was incorrect, showing how important it is to get a detailed inspection before replacing parts. Regular monitoring of sensor performance through diagnostic tools has shown everything is operating within normal parameters. Getting the correct diagnosis saved both time and money compared to unnecessary sensor replacement.

Since the initial diagnosis, I took my RS5 to a different mechanic who had better diagnostic tools. They found the faulty O2 sensor was affecting the air-fuel ratio and causing the poor fuel economy. The repair took about an hour and cost 320 Euro including parts and labor. The exhaust system works perfectly now. Fuel consumption is back to normal and the check engine light hasn't returned. The mechanic also checked the catalytic converter while they were working on it, confirming no damage occurred from the faulty sensor. This shop was much more thorough than my previous experience. They explained the entire diagnostic process and showed me the actual sensor readings before and after the repair. Really glad I switched mechanics, the difference in service quality was worth it.