Check Engine Light After Repair Points to ECU Lock Issue

Had a similar situation with my 2003 VW Polo TDI. The ECU fault memory lock became active shortly after routine diagnostics. The computer system showed an undervoltage condition that triggered the check engine light. The root cause was a failing voltage regulator in the alternator which wasnt supplying consistent power to the ECU. When automotive diagnostics are performed, the ECU becomes more sensitive to voltage fluctuations. This explains why these issues often surface after repair visits. In my case, the alternator needed replacement since the voltage regulator was integrated. The active memory lock was actually protecting the ECU from potential damage due to unstable voltage. While its a serious issue that needs attention, its not an emergency if the car still runs normally. The timing after your repair visit makes sense, diagnostic equipment can sometimes reveal existing weaknesses in the electrical system that were previously unnoticed. Would recommend getting the alternator output tested and having a full electrical system check. This will prevent potential ECU damage which would be far more costly to repair. Based on your vehicles age and mileage, its likely time for some electrical system maintenance anyway. The symptoms match classic voltage regulator failure patterns in these VW diesel models.

Thanks for sharing your Polo experience. Your explanation about the voltage regulator makes total sense. This gives me hope its a similar alternator issue and not something worse with the control unit. Can I ask what the repair cost was in your case? I want to budget accordingly. Also curious if youve had any related electrical problems since getting it fixed? My Lupo still drives fine but that check engine light is making me nervous, especially after reading about potential ECU damage. Think Ill get it checked this week while the auto repair shop can still connect it to the previous service visit.

Thanks for asking about the repair costs. Yes, my voltage regulator issue was resolved and the fault memory lock active warning cleared completely after the repair. The total came to 240€ for diagnosing and fixing the undervoltage condition in the computer system. Since the auto repair was completed 8 months ago, the ECU has been functioning perfectly with no recurring electrical problems. The check engine light hasnt returned, and the active memory has remained stable. The technician explained that catching these voltage issues early actually prevents more expensive damage to the electronic control systems. You are making a smart move getting your Lupo checked quickly. The symptoms you describe match exactly what I experienced, and prompt attention to ECU fault warnings typically leads to simpler, less costly repairs.

Just got back from the auto repair shop and wanted to update. Turned out to be exactly what you described, a failing voltage regulator. The automotive diagnostics confirmed the undervoltage issue in the control unit. The repair cost was slightly higher at 280€ but definitely worth it to protect the ECU from damage. The technician showed me the readings from the computer system before and after, what a difference. The active memory lock cleared right after they installed the new regulator. Really glad I caught this early. The mechanic mentioned they see this issue often in older VWs and fixing it promptly prevents bigger electrical problems down the road. The check engine light is now off and the car runs smoother than before. Thanks again for helping me understand what I was dealing with. Your experience helped me approach the repair shop with better knowledge of the likely issue.