VW Caddy Glow Plug Control Fault Causes Engine Issues

I had almost the same issue with my 2008 VW Touran diesel last winter. Having some experience with basic car maintenance, but electrical issues are definitely not my strong suit. The symptoms matched exactly, engine preheat light stuck on, check engine light, and similar smoke issues during acceleration. After attempting basic checks like looking for loose connections, I took it to a workshop since vehicle heater problems can be complex. The mechanic found the root cause quickly, a damaged wire connection to the control unit that manages the preheating system. The cable insulation had worn through, causing intermittent electrical contact. Total repair cost was 95€ including diagnostic work and fixing the defective part. Since the repair, the car electronics have worked flawlessly with no more warning lights or smoking issues. The engine preheat cycle now operates normally. To provide more specific advice for your Caddy: Could you share the engine size and whether you have access to diagnostic tools? Also, have you noticed if the problem gets worse in cold weather? This information would help determine if your issue is similar to what I experienced.

Thanks for sharing your experience. My Caddy had its last service at 89000km about 3 months ago, and the heating module was working fine then. The issue seems similar but more severe than what you described. The ignition system works normally, but the smoke is quite heavy under acceleration. When using diagnostic tools, I found the electrical fault code P0683 which points directly to the preheating unit circuit. The smoke pattern and constant warning light suggest there might be a complete failure of the glow plug control rather than just a loose connection. Noticed the issue is equally present regardless of engine temperature. Would you know if your workshop did a full diagnostic scan before replacing the wiring? Trying to determine if I should get the entire heating module replaced or just check the wiring first.

Thanks for those additional details about your Caddy. Your symptoms sound more severe than what I experienced with my Touran, especially that P0683 code which I didnt see in my case. Yes, my workshop did perform a complete diagnostic scan first. The wiring issue was relatively simple to spot once they had the scan results. But your case sounds different, the consistent smoke regardless of temperature and that specific fault code pointing to the preheating unit circuit suggests a more serious issue with the car preheater system itself. Looking back at my repair experience and comparing it to your symptoms, I would strongly recommend getting the complete heating module checked. While my issue was resolved with just a wiring repair, your electrical fault pattern indicates potential failure of the entire control unit. The heavy smoke during acceleration combined with the permanent preheating lamp typically means the glow plugs are either not working or getting incorrect signals from a defective part in the control system. This matches with what I learned during my repair process, though my issue was less severe. Given these differences, would suggest bypassing the wiring check and going straight for a heating module inspection. This might cost more than my 95€ repair, but could prevent potential engine damage from continued improper preheating.

Thanks for the helpful input. I have decided to book the car in for a full diagnostic check next week. The severity of the electrical fault and preheating issues combined with that P0683 code has convinced me not to delay this repair any longer. Will have them focus on the complete heating module rather than just checking wiring connections. The consistent smoke output and permanent warning lights regardless of temperature suggest this is the right approach. My local workshop quoted 280€ for the diagnostic work and potential heating module replacement. This seems reasonable considering the complexity of the ignition system and the specialized testing equipment needed. Will update once I know the exact cause and repair outcome. Hoping this helps others who might face similar vehicle heater problems with their diesel engines in the future.