VW Fox Diesel Glow Plug Control Unit Problems & Fixes

I had a very similar issue with my VW Polo diesel from 2003. From my experience with diesel engines, these symptoms typically point to glow plug system problems. The check engine light and constant preheating light were identical symptoms to what you describe. After running a diagnostic test at my local workshop, they found a faulty cable connection to the glow plug control unit. The smoke during acceleration was also present in my case. The repair involved checking all connections and replacing a damaged wiring harness that connected to the control unit. The total cost was 95€ for parts and labor. Since the fix, the vehicle stalling and smoking issues completely disappeared, and both warning lights went off. Before suggesting more specific solutions, could you provide: Current mileage, When was the last glow plug replacement, Any other warning lights showing, Does the smoke have a specific color, Does cold start performance seem affected This information would help determine if your issue matches what I experienced or if there might be other underlying problems with the glow plug system.

Last service was 2 months ago at 39075km. I want to add that the smoke is black and appears mainly during cold starts, which wasnt mentioned earlier. The glow plugs have never been replaced since I bought the car. No other warning lights are present besides the ones mentioned. The preheating light behavior seems different than in your case, it blinks several times before staying constantly on. The cold start performance has definitely worsened over the past weeks, especially during colder mornings. The heating element in the glow plug system might indeed be the culprit, but Id prefer to get a proper diagnostic test done before replacing parts. Based on your experience with the wiring harness issue, Ill make sure they check the connections thoroughly during inspection.

Thanks for the additional details. The blinking preheating light before staying on actually points more toward individual glow plug failure rather than the wiring harness issue I initially experienced. The black smoke during cold starts, combined with poor cold start performance, strengthens this theory. In my case, the smoke was more grayish, which was why the wiring diagnosis made sense. After your description, Id lean toward failed glow plugs rather than just the control unit. When I had similar symptoms develop later in my VW (at around 45000km), a diagnostic test revealed two failed glow plugs. The repair cost was 180€ for replacing all four glow plugs as preventive car maintenance. The blinking pattern of the heating element indicator is a key difference from my first issue. Multiple blinks often indicate the system detecting individual glow plug failures rather than a complete control unit malfunction. A proper diagnostic test should quickly identify which glow plugs are failing. Since yours have never been replaced, doing all of them at once might be cost-effective, especially considering the age of the car and the cold start issues youre experiencing.

Thanks for all the feedback. The blinking preheating light behavior does seem quite different from your wiring harness experience. My diesel engine symptoms definitely align more with your second repair case involving failed glow plugs. Given that the original glow plugs are still in place after all these years, replacing the full set makes sense. The black smoke during cold starts and deteriorating cold weather performance really point toward heating element failures rather than just control unit issues. Ill schedule a diagnostic test to pinpoint exactly which glow plugs are failing. Better to address all of them at once rather than face repeated vehicle stalling issues as they fail one by one. The 180€ cost you mentioned for a complete glow plug replacement seems reasonable considering the age of the car. Really helpful to hear about both your repair experiences, the wiring issue versus actual glow plug failures. The different preheating light patterns and smoke colors helped clarify what Im likely dealing with. Will update once I get the official diagnostic results.