Audi S4 Power Loss: ECU Trouble Symptoms Explained

I had a comparable situation with my 2005 Audi A4 2.0T. Having some experience with engine control issues, I initially thought it was an ECU failure too. After running diagnostics, the workshop found that the main relay was defective, which caused similar symptoms to ECU problems. The faulty relay disrupted proper communication between the control system and engine components, triggering limp mode. The repair involved replacing the main relay and performing a system reset to clear the stored error codes. Total cost was 120 Euro including parts and labor. The car has been running perfectly since then. To provide more specific advice for your case, could you share: Complete error codes stored in the system, Whether the issues occur when the engine is cold or warm, If you notice any other electrical problems, Whether the check engine light appears immediately after starting These details would help determine if your S4 has the same underlying issue or if the ECU repair might be needed instead. Remember that intermittent engine control problems can have multiple causes, from simple relay issues to complex vehicle electronics failures. Getting proper diagnostics is crucial before replacing expensive components.

Latest service done at 189500km. After checking the responses, I see similar patterns but my symptoms are different. When warm, the engine runs smoother and the power loss is less noticeable. The error codes show P0300 and P0303 consistently, even after multiple system resets. The check engine light appears about 5 minutes after starting. The computer module diagnostics from my regular mechanic pointed to a faulty ignition coil, but replacing it did not solve the issue. The car still goes into limp mode during acceleration. The ECU repair might be unavoidable at this point, as all vehicle electronics tests show irregular voltage patterns in the engine management system. Have any S4 owners found a reliable fix for these specific error codes and symptoms?

Thanks for the detailed follow-up. Your symptoms with the S4 definitely differ from my A4 case where the relay was the culprit. Based on those error codes (P0300/P0303) and behavior you described, I dealt with a very similar situation on my S4 last year. Initially, my mechanic also replaced ignition coils, but the problems persisted. After extensive car diagnostics, we discovered corrupted firmware in the ECU causing irregular voltage outputs to the ignition system. The automotive electronics specialist confirmed the ECU was sending inconsistent signals when warm. The final fix required: Complete ECU repair and reprogramming, New ignition control module, System-wide diagnostic reset Total cost was around 850 Euro, but the car has performed flawlessly for 40000km since the repair. The warm-up behavior you mentioned is typical for ECU-related misfires, as the control system struggles more with heat. For S4s of our generation, the engine control units are known to develop these issues, especially when the car passes 180000km. The irregular voltage patterns you mentioned are a clear indicator that the ECU is failing to properly manage the ignition timing. Would recommend getting the ECU tested by a specialist in automotive electronics before proceeding with the repair, just to rule out any other potential causes.

Thanks for the input. After comparing the symptoms and reviewing the suggested fixes, I think I need to proceed with the ECU repair route. The engine performance issues match what others experienced, especially the warm-up behavior affecting the control system. I contacted a specialist who deals with automotive electronics and scheduled comprehensive car diagnostics next week. They quoted 900 Euro for complete ECU repair and reprogramming if needed. While expensive, it seems justified given the consistent error codes and failed attempts with simpler fixes. The computer module testing they offer should confirm if the voltage irregularities are indeed causing the ignition timing problems. Will update the thread once I have the final diagnosis and repair results. Really hoping this resolves the limp mode issues during acceleration. Quick question for S4 owners who got their ECU repaired, did you need to get any additional programming done after the fix? Just want to make sure I budget for all potential costs.