SQ7 Auxiliary Heater Failure: System Won't Start

As someone with moderate experience working on Audi vehicles, I faced a very similar issue with my 2020 Q7. The auxiliary heater completely stopped responding, showing error codes while the main climate control worked fine. The root cause turned out to be a defective glow plug, which was preventing the auxiliary heater from initiating properly. The temperature sensor readings were normal, and the blower motor was functioning correctly, but the system would not activate due to this faulty component. The total repair cost was 440 Euro, including diagnostics, parts, and labor. My independent workshop completed the work in about 3 hours. They mentioned this is a somewhat common issue in this model family. To provide more specific guidance for your case, could you share: The exact error code displayed, Whether you hear any clicking sound when attempting to activate the auxiliary heater, If the issue started suddenly or developed gradually, Whether you notice any coolant leaks around the auxiliary heater unit This information would help determine if your issue matches what I experienced or if it might be something different.

Thanks for your comprehensive feedback. The ventilation system throws error code P0AC7 with no clicking sound when activating. The issue started last week and appeared suddenly. I checked around the auxiliary heater unit and found no coolant leak. The temperature sensor might indeed be the culprit. My SQ7 has 91126 KM and had its last service 3 months ago. Before taking it to the dealer, I wanted to check if changing the control module could resolve the issue, though your glow plug suggestion sounds like a more likely cause. Would checking the heat control unit be worthwhile before proceeding with repairs?

Thanks for providing those additional details. Based on the error code P0AC7 and symptoms you described, this does sound quite similar to what I encountered, though my experience suggests its likely not the control module. In my case, I initially suspected the control unit as well, but after diagnostic testing, the glow plug was definitively identified as the failure point. The absence of clicking sounds when activating matches exactly what I experienced before the repair. The heater core and auxiliary heating system are integrated in a way that makes testing the control module alone quite challenging. When my independent workshop attempted this, they found it more time and cost effective to perform a complete diagnostic sweep of the auxiliary heater components. Would strongly suggest having both the temperature sensor and glow plug checked first, as these are typically the more common failure points with our models. My mechanic explained that these components often fail without warning, regardless of mileage or service history. One key difference from your case, mine started with intermittent function before complete failure. Since yours failed suddenly, it might be worth having them inspect the wiring harness to the auxiliary heater as well. For reference, the diagnostic testing alone cost me 120 Euro before proceeding with the actual repair. This proved worthwhile as it prevented replacing unnecessary parts.

I appreciate the detailed analysis. After gathering all this information and comparing symptoms, I agree that checking the glow plug is the logical first step. The heating system in these models does seem prone to these specific failures. Given the sudden onset and complete loss of function in my auxiliary heater, along with that specific P0AC7 code, scheduling a proper diagnostic makes the most sense. While I initially thought the heat control unit might be the issue, the similar experiences shared here point strongly toward the glow plug assembly. The absence of any coolant leak and normal function of the main climate control system further supports this theory. I will schedule an appointment and request they specifically examine the glow plug and temperature sensor first, rather than jumping straight to more expensive components like the control module. At 440 Euro for a complete repair, this seems reasonable compared to what I was anticipating. Will proceed with the diagnostic testing and update once I have more information about the exact cause.