S8 Temperature Sensor Nightmare: Cooling System Alert

I had a similar issue with my 2020 Audi S7, and it turned out the HVAC system control unit was completely defective. After several inconsistent temperature readings and AC malfunctions, I took it to a workshop for sensor calibration testing. The technicians found that the control unit was sending incorrect signals to both the temperature sensor and heat exchanger components. The cooling system would out of nowhere shut down, triggering those error messages you mentioned. The repair involved replacing the entire AC control unit and recalibrating the temperature sensors. Total cost was 980 Euro, but it solved all the issues completely. No more error messages or inconsistent readings since then. Before suggesting specific fixes, could you share: Current mileage on your S8, When did these symptoms first appear, Any recent work done on the cooling system, Specific error codes showing up, Does it happen more during cold starts or when the engine is warm This information would help determine if your issue matches what I experienced or if it might be something different.

I am at 63518 KM since my last service in March. The temperature sensor issues started showing up about 2 weeks ago, first with inconsistent gauge readings, then progressing to complete HVAC system failures. No previous work was done on the cooling system. The error code P0128 keeps appearing consistently, especially during warm engine operation. The heat exchanger seems to be working intermittently, but the temperature gauge fluctuates wildly between normal and high readings. I suspect this could be more than just a sensor malfunction given the progressive nature of the symptoms.

Thanks for providing those details. Your symptoms actually match my S7 case quite closely, especially the P0128 code and progressive nature of the failures. However, after seeing your mileage and timing details, I have a different theory than my initial control unit diagnosis. In my follow-up experience with the vehicle, I discovered that temperature sensor malfunctions can often cascade into broader HVAC system issues. The intermittent heat exchanger operation you mentioned is particularly telling, this exact pattern occurred in my car about 2 months after the initial repair. The vehicle maintenance records showed that this specific sensor malfunction tends to trigger false readings in the temperature gauge, which then causes the control unit to make incorrect adjustments. While my initial fix focused on the control unit, I later learned that addressing the temperature sensor itself could have been a more cost-effective first step. Given your specific symptoms and error codes, I would suggest: 1. Have the temperature sensor tested separately before replacing the entire control unit 2. Check the wiring connections to the sensor, as heat cycling can cause deterioration 3. Consider a coolant system pressure test to rule out other issues This approach could save you from the 980 Euro repair I initially went through if the issue is indeed just the sensor.

Based on your insight and comparing it with my S8s symptoms, the temperature sensor testing seems like the most logical first step. Its helpful to know you initially replaced the entire control unit, when a more targeted fix might have worked. That matches what I was suspecting about my coolant leak and cooling system behavior. The intermittent heat exchanger operation youre describing sounds exactly like what Im experiencing. Good catch on checking the wiring connections too, I hadnt considered that heat cycling could be affecting the sensor malfunction patterns. Ill schedule a diagnostic test focusing specifically on the temperature sensor before considering a full control unit replacement. The progressive nature of my issues does align with your experience, but taking the more targeted troubleshooting approach could save me from an unnecessary 980 Euro repair. Thanks for sharing those repair details. Ill update the forum once I get the sensor testing results and confirm whether its just a sensor issue or something more complex in the cooling system.