Audi SQ7 Diesel Preheater Failure Symptoms Explained

Having some experience with diesel engines, I encountered a similar issue with my 2018 Audi Q7 diesel. The symptoms were nearly identical, continuous preheating lamp, check engine light, and smoke issues during acceleration. The electrical fault turned out to be a damaged cable connection to the engine preheat control module. The workshop diagnosed this using diagnostic equipment that showed error codes pointing to the car preheater system. The defective part was a wiring harness connector that had corroded over time. The fix involved replacing the damaged connector and resealing the connection point to prevent future moisture intrusion. Total repair cost was 95 Euro including diagnostics and parts. Before suggesting specific solutions for your case, could you provide: Current mileage, Any recent repairs or maintenance, When the symptoms first appeared, Whether the issue occurs more frequently during cold starts, If there are any other warning lights active This information would help determine if your SQ7 is experiencing the same underlying issue or if there might be a different cause.

Thanks for the detailed input. My Audi SQ7 has 158720 KM on the clock with the last service completed just 3 months ago. The preheating unit actually started acting up about 2 weeks after that service. The car electronics also show a P0671 code which points to a potential electrical fault in the glow plug circuit. The issue happens consistently on every start, regardless of engine temperature. No other warning lights except the preheating lamp and check engine indicators. The vehicle heater seems to work normally despite these warnings. I will get it checked at a workshop soon, but your experience with the wiring harness issue is really helpful. At least the repair cost in your case sounds reasonable.

Thanks for the additional details about your SQ7. After reading the P0671 code information, I am now even more convinced your issue matches what I experienced with my Q7. The fact that your heating module shows issues right after a service actually makes sense, sometimes connections can be disturbed during routine maintenance. The consistent occurrence on every start, regardless of temperature, is exactly how my problem manifested. The engine preheat system was getting incorrect signals due to the damaged wiring, causing the preheating lamp to stay on continuously. Since you mentioned the P0671 code specifically, this further confirms my suspicion of an electrical connection issue rather than a completely defective preheating control unit. In my case, the workshop initially suspected a failed car preheater component, but diagnostic testing revealed it was just the wiring connection. I would suggest having them specifically check: The wiring harness connectors near the heating module, The ground connection points, The connector seals for moisture intrusion Given the similar symptoms and error codes, you might be looking at a comparable repair cost around 100 Euro, assuming its the same root cause. Just make sure they thoroughly check the connections before replacing any major components of the vehicle heater system.

Thanks for the helpful information about wiring connections potentially being the culprit. Given my P0671 code and the timing right after service, I think you might be right about disturbed connections during maintenance. I just scheduled an appointment for next week to have them inspect the preheating unit wiring harness and connections. The service advisor was quite interested when I mentioned the possibility of disturbed connections after the last service, especially given the electrical fault showing in the engine preheat system. Knowing that your repair only cost around 100 Euro gives me some peace of mind. I was worried I might need to replace the entire preheating control unit, which would have been much more expensive. Will make sure they check all the connection points and seals you mentioned before replacing any major components of the vehicle heater. Will update the forum once I get it diagnosed and repaired in case others run into similar symptoms with their diesel engines.