RS4 HVAC System Failure: Temp Sensor Diagnosis Guide

As someone with some experience working on Audi vehicles, I encountered a very similar issue with my 2018 RS5. The symptoms were nearly identical, temperature gauge errors and complete HVAC system failure. After multiple diagnostic attempts, the workshop identified a defective air conditioning control unit as the root cause. The temperature sensor readings were incorrect because the control unit was malfunctioning, causing the engine cooling system to report false values. The repair involved: 1. Full diagnostic scan 2. Testing all cooling system components 3. Verifying coolant levels and checking for coolant leaks 4. Replacing the air conditioning control unit 5. Recalibrating the new unit Total cost was 980 Euro including parts and labor. The repair resolved both the temperature gauge errors and restored full HVAC functionality. To provide more specific advice for your case, could you share: Current mileage on your RS4? Any warning lights besides the temperature error? When did the problem first appear? Have you noticed any coolant loss? Does the AC compressor engage at all? These details would help determine if your issue matches the sensor malfunction I experienced or points to a different problem in the cooling system.

Thanks for the detailed response. My RS4 currently has 90000 KM since the last service in June. No additional warning lights have appeared besides the temperature gauge error. There does not seem to be any coolant leak, and fluid levels are normal. The AC compressor does not engage at all, which matches the symptoms you described with the control unit issue. This gives me more confidence that we are dealing with a similar sensor malfunction rather than a more serious cooling system problem. Will schedule a diagnostic scan to verify if the control unit needs replacement. The 980 Euro cost estimate helps with planning the repair budget.

Thanks for providing those additional details about your RS4. The symptoms align perfectly with what I experienced, especially the non-engaging AC compressor and the normal coolant levels without any visible coolant leak. Since your mileage is similar to when my issue occurred (I was at 85000 KM), and the service timing matches up, I am even more convinced we are looking at the same control unit malfunction. In my case, the sensor calibration was completely off, causing the system to misread temperatures. One thing I learned during my repair, make sure the workshop performs a complete HVAC system diagnostic before replacing parts. In some cases, the control unit can be recalibrated without replacement, which could save several hundred Euro. After my repair was completed, I had them run a full cooling system pressure test to ensure there were no underlying issues that could affect the new control unit. The entire engine cooling circuit checked out fine, confirming the control unit was indeed the sole problem. Keep us updated on what the diagnostic scan reveals. Based on the matching symptoms and vehicle history, I expect you will find the same root cause.

Thanks for the input. After taking my RS4 to the workshop yesterday, the diagnostic scan confirmed a faulty control unit, matching exactly what was discussed here. The temperature gauge error disappeared after they replaced and calibrated the new unit. Total repair cost came to 960 Euro, slightly less than expected. They also performed a full cooling system pressure test which showed no issues with the engine cooling circuit. The HVAC system now works perfectly and all sensor readings are back to normal. For anyone else facing similar temperature gauge errors combined with HVAC failures in these models, definitely get the control unit checked first before exploring other potential issues. A good lesson learned about the importance of proper sensor calibration and diagnostic testing before replacing parts. Really glad it wasnt a more serious coolant leak or engine cooling problem.