S4 ECU Locked: Control Unit Memory Issues and Fixes

Had a similar issue with my 2017 S5 and can share some experience as someone with moderate auto repair knowledge. The symptoms match what I encountered, persistent fault codes and a locked control unit that wouldnt reset. The root cause turned out to be undervoltage, which was affecting the computer system and creating memory lock conditions that standard OBD readers couldnt clear. After unsuccessful attempts with basic diagnostics, took it to a workshop specializing in auto electrical systems. They used advanced diagnostic equipment to: 1. Stabilize the voltage supply 2. Perform a full control unit reset 3. Clear persistent fault codes from the memory 4. Recalibrate affected systems Total repair cost was 240 Euro and resolved the issue completely. No recurring problems since then. To provide more specific guidance, could you share: Battery age and condition, Recent electrical work or modifications, Specific fault codes showing up, Any other electrical symptoms (dim lights, slow starter) This would help determine if your issue matches what I experienced and whether the same fix might work.

Thanks for sharing that fix. After seeing your post, checked my maintenance records, last service at 21315 KM and battery tests were fine. The car electronics issue is definitely different from yours since no voltage problems show up in diagnostics. The ECU fault persists even with a fully charged battery and clean connections. Active memory seems permanently locked, blocking access to clear the codes. Multiple OBD scans just return the same locked state in the computer system. Wondering if anyone has dealt with this specific issue on the 2019 S4 platform? The persistent error codes point to a deeper control unit problem rather than a power supply issue.

Thanks for the additional details about your S4, this definitely points to a different root cause than my voltage-related experience. After your response, I did more research since my S5 eventually showed similar symptoms months later. The locked memory turned out to be a firmware issue in the control unit rather than a hardware problem. The automotive diagnostics revealed that certain ECU versions had a software conflict causing permanent memory locks. The fix required specialized dealer-level diagnostic equipment to: Force a complete ECU diagnostics reset, Update the control unit firmware, Recalibrate system parameters Cost was higher at 380 Euro but solved the persistent lock completely. The computer system now responds normally to standard OBD clearing attempts. Key difference between our cases: while my first issue was power-related, this second occurrence matches your symptoms much closer, especially the permanent memory lock despite good battery condition. Would recommend checking your ECU firmware version through a diagnostic scan. The control unit might need the same update that resolved my later issue.

Thanks for all the detailed insights. After digging deeper into the ECU diagnostics, I think I found the core problem. My fault codes show the same firmware conflict you mentioned, explains why the memory lock persists despite everything else checking out normal. Going to schedule service at a shop that has the right equipment for advanced car electronics work. Will make sure they can handle ECU firmware updates and full computer system resets before booking. The 380 Euro cost seems reasonable if it properly fixes these persistent fault codes and memory lock issues. Really helpful to know the control unit firmware was the culprit in your case rather than a hardware problem. Saves me from trying more basic fixes that likely wouldnt work. Will update once I get it resolved in case others run into similar ECU diagnostic issues with their 2019 models.