VW Fox Diesel Won't Start: ECU Issue Alert

Having worked on various VWs, I experienced a very similar issue with my VW Polo 2007 diesel. When the engine light came on and the car refused to start despite cranking, I initially suspected major engine software problems. The workshop diagnosed it as corroded battery terminals causing inconsistent power supply to the ECU. This triggered the engine control warning light and prevented proper startup. While the battery itself tested fine, the poor connections were causing engine surge and unusual engine stalling issues. The fix involved cleaning all battery connections, replacing corroded terminals, and clearing the stored error codes. Total cost was 155€ including diagnostics and labor. Before suggesting more specific solutions, could you provide: Current mileage, Any recent work done on the electrical system, Whether the check engine light stays on during cranking, If you notice any electrical issues like dim lights, Weather conditions when the problem started This information would help determine if your Fox has the same underlying issue or needs different diagnostics.

Thanks for the helpful reply. My mileage is 121226km and last service was done 2 months ago with standard maintenance including fuel injection system cleaning. The check engine light stays on during cranking and no other electrical issues are present. The weather was dry when it happened. Interesting point about the battery terminals, I will definitely check those connections more thoroughly as it sounds similar to my situation. The engine surge symptoms you mentioned were not present in my case though. Before spending on full engine diagnostics, I will try cleaning the connections first.

Thanks for the additional details. Given your response about the fuel injection cleaning during recent service, I should mention that my Polo actually showed similar symptoms again about 3 months after my initial battery terminal fix. The fuel system cleaning triggered a chain reaction, where residual debris caused engine control issues. The check engine light appeared and the engine refused to start, just like before. The workshop found that the fuel pressure sensor was giving false readings to the ECU, causing an engine fault condition. The fix required replacing the fuel pressure sensor and performing a complete ECU reset. Cost was higher this time at 320€, but it resolved all starting issues. Given your Fox has similar injection architecture, and the timing after your service work, you might want to have the fuel pressure monitoring system checked if cleaning the battery connections doesnt solve it. Looking at both our cases, when the engine light stays on during cranking without other electrical issues, its often more related to sensor inputs than actual ECU failure. This tends to be a much cheaper fix than replacing engine control modules. Have you noticed any changes in engine performance just before the service or immediately after the injection system cleaning?

Thanks for sharing those experiences. Looking at my service history, I did notice slightly rough idling for a few days after the fuel system cleaning, but thought it was normal and would settle. Now connecting the dots with your Polo case, this could indeed point towards sensor issues rather than a complete ECU failure. I checked all battery terminals and connections today, they were actually quite clean with no visible corrosion. Since this doesnt seem to be the culprit, I will schedule a workshop visit focused on checking the fuel pressure monitoring system. The 320€ repair cost you mentioned for sensor replacement and ECU reset sounds much better than what I feared for a full ECU replacement. Will make sure to ask them specifically about checking the fuel pressure sensor given the timing after my injection system service. This really helps narrow down the potential engine fault causes. Thanks again for sharing the repair details, it gives me a much clearer direction for the workshop visit.