RS4 ECU Issues Causing Major Performance Drop

Having dealt with a similar issue on my 2019 S4, I can share my experience as someone with moderate experience in car repairs. The symptoms you describe match what I encountered, reduced power and multiple error codes. Initially, the car diagnostics were inconclusive, but after a thorough system check, the workshop identified a defective main relay affecting the engine control unit communication. The repair involved replacing the main relay, followed by a complete system reset of the vehicle electronics. Total cost was 120 Euro, which included diagnostics and parts. Much better than replacing the entire computer module, which was initially suspected. Before suggesting more specific solutions, could you share: What error codes are you getting? Does the issue occur more frequently when the engine is cold or warm? Have you noticed any warning lights on the dashboard? Any recent work done on the electrical system? This information would help determine if your issue matches what I experienced or if its something different in the engine control system.

Thanks for detailing your experience. My RS4 with 48373 KM had its last service just 2 months ago. The error codes are P0685 and P0687, appearing consistently whether the engine is cold or warm. The check engine light stays on constantly, and no recent electrical work was done. The car diagnostics also showed voltage fluctuations in the engine control system. Based on your feedback about the main relay, this definitely sounds similar to my situation. Will have the workshop check the relay before considering a full computer module replacement.

Your symptoms match my S4 case even more closely now that you've shared those specific error codes. P0685 and P0687 are exactly what my car diagnostics revealed, they typically point to main relay control circuit issues in the engine control system. The voltage fluctuations you mentioned were also present in my case, which was ultimately traced to the degraded relay affecting the vehicle electronics. The fact that the issue persists regardless of engine temperature further supports this diagnosis, as faulty engine control unit behavior would typically show some variation. Since your RS4 has relatively low mileage and recent service history, a relay failure makes more sense than a complete computer module failure. In my repair experience, these automotive electronics issues often stem from simpler components rather than the main control unit. Some tips from my repair: Have them test the relay circuit under load, Request voltage readings at different connection points, Ensure all ground connections are clean and secure My repair lasted about 2 hours including all car diagnostics and testing. Given the matching symptoms and error codes, I strongly suggest trying the relay replacement first, it could save you from an unnecessary engine control unit replacement.

Thanks for the detailed input. I just got back from the workshop and wanted to close this thread with an update. You were right, it was indeed the main relay causing issues with the engine control system. The car diagnostics confirmed the voltage fluctuations were coming from the faulty relay. The repair cost was 140 Euro including parts and labor, plus they did a complete system reset of the automotive electronics. The check engine light is now off and power is back to normal. No more error codes showing up in the computer module either. Really glad I asked here before agreeing to replace the entire ECU. This saved me significant money and downtime. The workshop also cleaned all ground connections while they were at it, as suggested. For anyone searching with similar symptoms and error codes P0685/P0687, definitely check the relay first before considering more extensive repairs.