SQ8 TDI Heater Control Unit Causes Smoke and Warning

I faced a very similar electrical fault with my 2017 Q7 TDI. Having some experience with diesel engines, I recognized these symptoms immediately as they matched my situation from last winter. The constant preheating lamp and smoke issues were caused by a faulty connection between the preheating unit and its control module. The car electronics diagnosis revealed that the cable harness had deteriorated, causing intermittent connection problems. The workshop found that one of the plug connections was corroded, disrupting the proper function of the preheating system. The repair involved cleaning the connections and replacing the damaged wiring section. Total cost was 95 Euro including diagnostic work. The defective part was actually quite accessible once located, though proper diagnosis was crucial to avoid replacing parts unnecessarily. After the repair, both the preheating lamp issue and excessive smoke completely resolved. To provide more specific advice for your case, could you share: Have you noticed any starting difficulties, especially in cold weather? Did these symptoms appear gradually or suddenly? Are there any other warning lights active besides the ones mentioned? Has any previous work been done on the preheating system?

The last service was done at 65000 KM and I noticed these issues shortly after. The mechanic found a faulty heating module wire connection, but only performed basic diagnostics without a complete fix. The vehicle heater started acting up gradually over two weeks. Cold starts became increasingly difficult, taking 3-4 attempts before the engine would turn over properly. The ignition system seems particularly stressed during these attempts. The electrical fault appears to be getting worse, with the preheating lamp now staying on for longer periods. No other warning lights are active besides the ones mentioned in my original post. No previous repairs have been done to the preheating system before these recent issues started appearing. The smoke during acceleration has become more noticeable over the past few days. Looking into scheduling a proper diagnostic session to identify if this requires just rewiring or a complete heating module replacement. Based on your experience, I suspect the wiring might be the core issue here.

Thanks for providing those details about your SQ8. Your symptoms sound very familiar to what I experienced with my Q7. The gradual onset and worsening cold start issues strongly point to a deteriorating connection in the preheating system, similar to my case. The fact that these problems emerged after your service visit suggests the heating module wiring might have been disturbed during maintenance. When my electrical fault first appeared, I also noticed the symptoms getting progressively worse before addressing them. The control unit in these models is quite reliable, so before replacing the entire heating module, have the workshop thoroughly check the wiring connections. In my case, what initially seemed like a defective part turned out to be just corroded connectors affecting the vehicle heater functionality. Given the increasing smoke and starting difficulties, I would prioritize getting this checked soon. When my wiring was fixed, the complete repair process took about 2 hours. The smoke issues cleared up immediately after the repair, and the preheating lamp started functioning normally again. For proper diagnosis, make sure they specifically check: Wiring harness condition around the heating module, Connector pins for corrosion, Ground connections, Control unit signal output

Based on your detailed feedback, I will schedule the diagnostic session this week. Your experience with the wiring issue rather than a complete control unit failure gives me hope this might be a simpler fix than initially feared. The similar progression of symptoms, especially regarding cold starts and smoke, matches my situation perfectly. It makes sense that the service work might have disturbed the preheating unit connections, leading to these problems. Since the vehicle heater issues developed gradually and the car electronics seem otherwise fine, focusing on the wiring connections first appears to be the logical approach. I will request they specifically examine the points you mentioned, particularly the heating module wiring harness and connector pins. If the defective part turns out to be just corroded connections like in your case, this should be a relatively straightforward repair. I will update once the workshop completes their diagnosis and repair work. Really hoping to avoid a complete preheating system replacement, as these cold start issues are becoming quite frustrating.