Audi TDI Glow Plug Issues: Control Module Fix Guide

Had a very similar issue with my 2007 Audi Q5 3.0 TDI. As someone with moderate experience in car maintenance, I initially thought it was just a faulty glow plug, but it turned out to be more complex. The diesel engine symptoms you describe match what I experienced, constant preheating light, engine light on, and rough cold starts. After some basic troubleshooting, I took it to a workshop where they diagnosed a loose connection in the glow plug control unit wiring harness. The cable insulation had worn down over time, causing intermittent connection issues. The repair cost me 95 Euro, which covered diagnosing and fixing the faulty cable connection. After the repair, all symptoms disappeared and the engine has been running smoothly since. Before suggesting specific fixes, could you provide: Current mileage, When was the last glow plug replacement, Any diagnostic codes from the engine control unit, Do these symptoms occur more frequently in wet weather This information would help determine if your issue is identical to what I experienced or if there might be other factors at play. Also worth checking if there are any technical service bulletins for your specific model year regarding the glow plug system, as this can help narrow down the diagnosis.

Thanks for sharing your experience with the glow plug system issue. My vehicle currently has 149283 KM and just had its annual service last month. After reading about your wiring harness problem, I checked my engine codes using a basic scanner and got P0671 and P0684. The symptoms definitely get worse on damp mornings during cold start. The vehicle stalling has become more frequent too. The smoke during acceleration seems more pronounced than what you described. Given the diagnostic test results and the fact that my glow plugs are original, I am now leaning towards replacing both the glow plugs and the control module to solve this diesel engine issue.

Thanks for the additional details. Those diagnostic codes (P0671 indicating cylinder 1 glow plug circuit malfunction and P0684 showing glow plug control module issues) definitely paint a clearer picture than what I initially experienced. The excessive smoke during acceleration combined with stalling suggests your issue is more severe than my faulty wiring. The engine misfire symptoms, especially during cold start, point strongly toward multiple failing glow plugs rather than just a control module problem. In my case, the repair was simpler because it was just the wiring. Given your car still has the original glow plugs at 149K, replacing both glow plugs and the control module is a smart approach. The total repair on a 3.0 TDI typically runs around 450 Euro for parts and labor, but it should resolve all the symptoms completely. Quick tip from my experience: make sure the workshop performs a full diagnostic test after the repair to confirm all circuits are functioning correctly. When I had mine fixed, they initially missed testing all circuits, and one glow plug still had connectivity issues.

Thanks for the comprehensive feedback. After reviewing your case and comparing it to mine, I will proceed with the complete glow plug system overhaul. The information about getting a full diagnostic test afterward is particularly valuable, I will make sure to request this specifically at the workshop. The fact that your repair involved just the wiring but mine shows multiple fault codes makes me more confident about replacing both the heating elements and the control unit. Given the age of my engine and the original glow plugs, this preventive maintenance approach makes sense. Will book the repair next week and expect to pay around 450 Euro for the complete job. The chronic cold start issues and engine light warnings have become too problematic to ignore, especially with winter approaching. Thanks again for sharing your experience, it helped confirm my diagnosis and plan of action for fixing these diesel engine problems.