S5 Diesel Preheat System Fault Causing Heavy Smoke

As an owner of a 2018 S4 diesel, I encountered nearly identical symptoms last winter. The engine preheat warning remained lit, and I also noticed excessive smoke under acceleration. After some basic troubleshooting attempts, I took it to a workshop where they diagnosed the issue. The problem turned out to be a damaged wiring connection to the preheating unit. The heating module was functioning correctly, but the control unit wasn't receiving proper signals due to a compromised cable. The repair involved replacing the affected wiring harness and ensuring proper connections to the preheating system components. Total cost was 95 Euro for parts and labor. Since the fix, the engine preheat system has worked flawlessly, with normal cold starts and no more warning lights or smoke issues. The preheating unit now cycles normally during cold starts. To provide more specific guidance for your case, could you share: Current mileage, When the symptoms first appeared, Whether the issue occurs more during cold or warm weather, If any other warning messages appear on the dashboard, Whether you notice hard starting, especially in cold weather

My last service was at 50000 KM, about 2 months ago. Since then, I have driven 3603 KM. The vehicle preheater symptoms appeared about a week ago, and the issues persist regardless of engine temperature. A diagnostic scan reveals error codes related to the preheating unit malfunction. I checked the wiring connections based on your feedback, but they appear intact. The preheating lamp remains constantly illuminated, and the smoke during acceleration continues. The ignition system seems affected as well, with occasional rough starts even in mild temperatures. Perhaps the car electronics need a deeper inspection focusing on the control module itself rather than just the wiring. This seems different from your wiring harness issue, as the symptoms occur consistently in all weather conditions.

Thanks for those additional details. Your situation does sound different from my wiring harness experience, particularly since your symptoms persist regardless of temperature. The consistent preheating lamp illumination combined with starting issues points more toward a defective control unit or glow plugs rather than wiring. The diagnostic error codes specific to the preheating unit malfunction strengthen this theory. In my subsequent research after my own repairs, I learned that when the vehicle heater system shows these symptoms across all conditions, its often tied to the control unit itself failing rather than connection issues. Given that this started after your recent service, consider having the workshop specifically test: 1. The glow plug control unit functionality 2. Individual glow plug resistance values 3. Power supply to the preheating system Testing these components individually should pinpoint whether its the control unit or the glow plugs themselves. Based on similar cases in our model range, replacement costs typically run 200-350 Euro for the control unit if thats confirmed as the failure point. Would you mind sharing the specific error codes from your diagnostic scan? This could help narrow down whether its the engine preheat electronics or possibly a related sensor issue.

Thanks for the comprehensive diagnostic suggestions. The error codes from my scan tool show P0380 (Glow Plug/Heater Circuit A Malfunction) and P0670 (Glow Plug Control Module Circuit). After reviewing your feedback, I scheduled an appointment to have the control unit tested. You make a good point about checking individual glow plug resistance values, as the electrical fault could be affecting multiple components in the heating module system. The smoking during acceleration and constant preheating lamp definitely align with the control unit theory rather than just a wiring issue. The ignition system behavior and error codes paint a clear picture of a failing preheating control module. I will update once the workshop completes their testing. The quoted price range of 200-350 Euro for a control unit replacement seems reasonable considering the symptoms and diagnostic evidence. Getting the entire system properly tested should prevent replacing parts unnecessarily.