VW T4 Control Unit Lock: ECU Protection Fault Alert

Had a similar situation with my VW T5 2.0 TSI from 2012. The automotive diagnostics showed eerily similar symptoms, multiple codes and ECU fault memory locks that kept reappearing after clearing. After some failed attempts at fixing it myself (moderate experience with car electronics), I took it to a workshop. They found the root cause was undervoltage in the vehicle electrical system. The computer system was going into protection mode because the voltage was dropping below critical levels. The fix cost 240€ and included: Complete voltage system testing, Battery load test, Alternator output verification, Ground connection cleaning and restoration, ECU reset after fixing the voltage issue The auto repair solved it completely, no more error codes or lockouts since then. To help diagnose your case more accurately, could you share: The exact year of your T4, Which error codes are showing up, Whether you notice any electrical issues (dim lights, slow starter), If the battery is original or when it was last replaced, Whether the alternator has ever been replaced This information would help determine if your issue matches what I experienced with my vehicle.

Last service was done at 68838km, and I can confirm this is still an ongoing issue. The fault codes reappear regardless of battery voltage readings. Had the alternator tested and it puts out correct voltage. Power and ground connections appear solid based on multimeter checks. This computer system lockout seems more complex than a basic voltage issue since even a fully charged battery doesnt resolve it. The automotive diagnostics point toward something corrupted in the control module itself. The memory lock state persists through multiple key cycles and seems to prevent any meaningful auto repair attempts. Need an actual solution to unlock the ECU to perform proper troubleshooting. Has anyone successfully dealt with a persistent fault code memory lock without replacing the entire control unit?

Thanks for the additional details. After dealing with practically identical symptoms on my T5, I also initially thought my alternator and charging system were fine based on basic voltage readings. However, the fault memory lock and ECU fault patterns turned out to be more complex than steady voltage measurements suggested. The automotive diagnostics eventually revealed voltage micro-drops during specific operating conditions that were triggering the active memory protection mode. What helped diagnose this accurately was: Using an oscilloscope to monitor voltage patterns, Testing under various engine loads, Checking voltage stability during ECU communication The memory lock mechanism exists to prevent control module damage. While the voltage looked good on a basic meter, the computer was detecting unstable power that standard testing missed. In my case, cleaning all ground points and adding a ground strap resolved the micro-drops completely. The ECU returned to normal operation without needing replacement. Total fix was around 180€ including diagnostic time. Before replacing the control unit, suggest having a shop: 1. Monitor live voltage data during ECU communications 2. Test for voltage stability under load 3. Verify all ground connections with voltage drop testing 4. Attempt a full power reset procedure The active memory protection is likely trying to tell us something about power stability that basic testing isnt catching.

Thanks for the detailed responses so far, but I still feel like were missing something with my T4 issue. After checking the car today, I found more clues: The active memory lock happens even when the ECU diagnostics are run with an auxiliary power supply connected. This rules out any vehicle power system problems since the computer system is getting clean stable power during testing. I did notice the fault codes change slightly between scans, but the locked state remains constant. This suggests internal ECU corruption rather than external triggers. The automotive diagnostics also show inconsistent parameter readings when monitoring live data. Really hoping someone has experience with similar control module software corruption and found a fix that doesnt require full replacement. A 700€ new unit would be my absolute last resort since the car runs decent otherwise despite being stuck in this locked fault memory state. Appreciate any suggestions for recovering ECU function without replacing hardware. There must be some kind of reset or reflash procedure that can resolve persistent memory locks caused by software issues rather than power problems.