VW Golf TDI Glow Plug System Failure Warning Signs

I had a similar issue with my 2010 VW Passat TDI. Based on some experience working with diesel engines, these symptoms definitely sound familiar. My preheating unit was showing identical warning signs, constant preheating lamp, check engine light, and the smoking during acceleration. The electrical fault turned out to be exactly what you suspect, a damaged cable connection to the control unit of the vehicle heater system. The workshop diagnosed it by running a complete ignition system check. They found that moisture had corroded the plug connection to the preheating control unit. The fix involved replacing the wiring harness and connections, totaling 95€ for parts and labor. Before jumping to conclusions though, could you share: Have you noticed any hard starting, especially in cold weather? Does the engine shake or run rough when first starting? What error codes is your system showing? Has there been any recent work done on the ignition system? This information would help determine if your issue matches what I experienced or if there might be other factors involved with your preheating unit.

Last service was 6 months ago at 52355km. The engine shakes noticeably during cold starts and takes longer to turn over. I have noticed harder starting in colder temperatures recently. My diagnostic reader shows error code P0670 which points to the heating module circuit. No recent work has been done on the ignition system. The engine preheat symptoms started gradually over the past few weeks. At first it was just occasional rough starts, but now the preheating lamp stays on consistently. I did check the wiring connections like you suggested but they appear clean with no visible corrosion. Since the car preheater issues seem worse than what you experienced, I suspect it might require replacing the entire defective part rather than just the wiring.

Thanks for those additional details. Having gone through this with my Passat TDI, your symptoms point to a more serious issue than what I initially dealt with. The error code P0670 combined with the progressively worsening cold starts suggests the heating module itself might be the defective part, not just the wiring. Your mileage is actually similar to when my car preheater system failed completely. While my issue was resolved with wiring repairs, your pristine connections indicate the engine preheat control unit has likely failed internally. The consistent preheating lamp and rough starting behavior match classic symptoms of a failing control module. I ended up having additional problems a few months after the initial wiring fix, and ultimately needed to replace the entire heating module. The complete repair including parts and labor ran about 450€. The workshop also recommended checking the glow plugs while doing this work, as a faulty ignition system component can sometimes damage them. Quick tip: Until you can get it repaired, try to avoid short trips in cold weather as this puts extra stress on the engine preheat system. The constant rough starts could potentially cause additional issues. Have you noticed any change in fuel consumption since these symptoms started?

Thanks for your detailed response. Yes, actually I have noticed about 15-20% higher fuel consumption in the last few weeks since these car electronics issues began. Really appreciate the tip about avoiding short cold trips, I will definitely keep that in mind until I can get this fixed. Based on what you described, I will schedule an appointment to have the full preheating unit replaced rather than attempting just a wiring repair. The combination of the P0670 code, rough cold starts, and increased fuel usage matches the symptoms you experienced before needing the complete heating module replacement. The 450€ repair cost you mentioned helps set my expectations. I will also ask them to inspect the glow plugs while they have the vehicle heater system apart, since that seems like a smart preventive measure. Better to address any potential ignition system damage now rather than face additional problems in a few months like you did. Thanks again for sharing your experience. This really helped clarify what I am likely dealing with and the best path forward for repairs.