VW T-Cross Performance Drop Linked to ECU Problems

My 2021 T-Roc had almost identical vehicle electronics issues last summer. Having some experience with VW engine performance problems, I first suspected the ECU, but it turned out to be simpler. After car diagnostics at the workshop, they found the main relay was defective, causing intermittent power supply issues to the ECU. This explained the reduced engine performance and error codes. The repair involved replacing the main relay, followed by a system reset to clear the stored error codes. Total cost was 120€ including parts and labor. The fix restored normal engine performance immediately. No ECU replacement was needed, which would have been much more expensive. Before suggesting more specific solutions, could you share: Current mileage, Any warning lights on the dashboard, Whether the issues occur more when the engine is cold or warm, If you notice any other electrical problems, What specific error codes were shown in the diagnostics This information would help determine if your T-Cross is experiencing the same relay issue or if ECU repair might actually be needed.

Last service was at 70000km and current mileage is 72516km. The workshop I took it to found a P0303 code indicating cylinder 3 misfires and P0171 suggesting a lean fuel mixture. No dashboard lights except the check engine light which started blinking occasionally during acceleration. The engine performance issues happen consistently regardless of temperature, but the control system seems most erratic during highway speeds. The computer module might be getting incorrect readings from sensors since fuel consumption has also increased noticeably. I checked the electrical connections as suggested but found no obvious issues. I will definitely have them look at the main relay based on your experience. The ECU repair costs quoted seemed excessive, so hopefully it is something simpler like in your case.

Thanks for providing those details. After fixing my T-Roc's relay issue, I learned a lot about automotive electronics and similar symptoms. The error codes you mentioned point to a different root cause than what I experienced. The combination of P0303 and P0171 codes typically indicates ignition system problems rather than a failing engine control unit. The erratic behavior during highway speeds and increased fuel consumption matches my initial symptoms, but your misfiring indicates you should check: 1. Spark plugs and ignition coils, especially for cylinder 3 2. Mass airflow sensor readings 3. Fuel injector performance 4. Vacuum leaks in the intake system The control system might be functioning correctly but responding to faulty sensor inputs or failing components. A system reset after fixing the underlying mechanical issue should clear the stored errors. The workshop should perform a complete diagnostic of the ignition system before considering expensive ECU repairs. In my case, even though the symptoms seemed like ECU problems, the actual fix was much simpler. The blinking check engine light during acceleration strongly suggests ignition-related issues rather than a computer module failure.

I really appreciate this detailed response about the potential ignition system issues. Seems like my initial concerns about the ECU might have been off track. The engine performance problems do align with what you described regarding ignition coils and spark plugs, especially with that blinking check engine light during acceleration. Makes sense that the control system could be working fine but just responding to bad sensor data or failing components. Will have the workshop check all those points you listed, particularly focusing on cylinder 3 since that's where the misfire code is coming from. Good to know the automotive electronics might not be the primary issue. I feel more confident now about avoiding unnecessary ECU repair costs by having them investigate the ignition system first. Will update once the workshop completes their diagnostic testing of the components you mentioned. Thanks for sharing your experience, it helped narrow down the likely causes and saved me from potentially expensive repairs that might not have addressed the real problem.