SQ5 Diesel Glow Plug Issues Cause Smoke and Warnings

Had nearly identical issues with my Q5 2018 Diesel last winter. Have some experience working on basic car maintenance, but this was definitely beyond DIY territory. The control unit was showing similar symptoms, constant preheating lamp and check engine warnings. The vehicle heater would act up especially during cold starts. After diagnosing, the workshop found a damaged cable connection to the heating module, which was causing inconsistent signals to the ignition system. The fix involved replacing the wiring harness connected to the preheating unit. Total repair cost was 95 Euro, mainly labor since the issue was just the connection rather than the component itself. Before you head to a workshop, could you share: Current mileage on your SQ5? When did these symptoms first appear? Does the problem occur more frequently during cold starts? Have you noticed any changes in fuel consumption? This info would help determine if you are dealing with the same root cause. The heating module issues can sometimes be linked to other components in the ignition system, so these details would be helpful.

Mileage is at 195761 KM and last service was performed just 2 months ago. The symptoms started appearing gradually over the past 2 weeks. Yes, the issue is more noticeable during cold starts, and the engine preheat cycle seems to take longer than usual. The car preheater warning appears consistently now, not just during startup. Fuel consumption has increased about 15% based on my calculations. The electrical fault indicators are more frequent during morning starts, but the issue persists throughout the day. Thank you for sharing your experience. The relatively low repair cost in your case is encouraging, though Im concerned my heating module might need a complete replacement rather than just a wiring fix. Will get it checked at the workshop this week.

Those symptoms definitely align with what I experienced. Given your mileage and the gradual onset, I still lean towards a wiring issue rather than a complete control unit failure. The extended preheating cycle combined with increased fuel consumption matches exactly what happened with my Q5. The electrical fault pattern you describe during cold starts strongly suggests connection issues in the car preheater system. While my repair only needed wiring fixes, a full vehicle heater diagnostic would be wise at your mileage. Even if the control unit itself needs replacement, catching it early prevents damage to other ignition system components. My mechanic explained that the smoking during acceleration often happens because the system compensates for poor preheating by injecting extra fuel. This matches your 15% consumption increase. Quick tip from my experience: when you take it in, ask them to check the connector pins on both the wiring harness and control unit side. Sometimes oxidation at the connection points can mimic a complete unit failure. My total repair time was about 4 hours, but that was for a simple wiring fix. Budget for potentially longer if they need to replace the complete heating module.

Thanks for the detailed feedback and tip about the connector pins. The 15% fuel consumption increase was actually an underestimate, just calculated again and its closer to 18% over the past 2 weeks. The engine preheat cycle this morning took almost 45 seconds, which is definitely not normal. While your wiring harness issue gives me hope for a simpler fix, Im preparing for a potential full heating module replacement given the complete system behavior. The car preheater warning now stays illuminated even after the engine is fully warmed up, which seems different from your experience. The electrical fault messages are also more frequent than what you described. Going to the workshop tomorrow morning, will request they start with a thorough diagnostic of the connectors and wiring before replacing any major components. At least the timing is decent since its part of my routine maintenance schedule anyway. Will update the forum once I have a proper diagnosis and repair estimate from the workshop.