Audi TT Cooling Sensor Issues Causing AC Problems

Had a similar problem with my 2004 Audi S3 1.8T a few years back. Have some experience with basic vehicle maintenance, but needed professional help for this one. The cooling system was showing similar symptoms, AC not engaging and dashboard warnings. Initially thought it was just a temperature sensor acting up, but my workshop found the root cause was actually a defective AC control unit. The heat exchanger and its sensors tested fine during diagnostics. The repair involved: Full diagnostic scan, Testing all cooling system components, Replacing the AC control unit, Recalibrating the system, Recharging AC refrigerant Total cost came to 980 Euro including parts and labor. Since the fix, no more issues with the AC or error messages. To help further with your specific case, could you share: Any specific error codes showing up? Does the AC work intermittently or not at all? Have you noticed any changes in engine temperature readings? Was there any recent cooling system service done? This sensor malfunction could point to different issues depending on these details. The TT and S3 share many cooling system components, so comparing symptoms might help narrow down the problem.

Thanks for the detailed response. Last service was at 75520km and included basic fluid changes. The AC doesnt work at all, and error code P0116 keeps showing up. Engine temperature readings fluctuate out of nowhere on the dash display. No recent cooling system work besides regular coolant checks. Your sensor calibration steps are helpful, though my symptoms sound different since my temperature sensor seems to be the core issue rather than the AC control unit. Will get the heat exchanger sensor tested first before looking at other components. The diagnostic approach you outlined makes sense as a starting point.

Interesting follow-up about your P0116 code. Looking back at my S3 repair experience, I initially focused too much on the AC control unit, but your symptoms do point more toward a temperature sensor malfunction. Those fluctuating temperature readings are a classic sign. The engine cooling system on these 1.8T engines can be tricky. After dealing with my AC issues, I learned that unusual temperature fluctuations often indicate a failing sensor before showing other problems. A faulty heat exchanger temperature sensor can definitely trigger both AC shutdown and erratic gauge readings. Quick tip from my experience: make sure to check for any coolant leak near the sensor connection. My initial sensor issue actually masked a small leak that was causing intermittent electrical interference. Cost reference for comparison: Temperature sensor replacement: 120 Euro, Diagnostic testing: 80 Euro, Labor: 100 Euro Would recommend getting the coolant system pressure tested while they are checking the sensor. This helps rule out any hidden issues that could damage a new sensor.

Thanks for sharing those details about the sensor and AC issues. The P0116 code definitely helps confirm my initial suspicion about the temperature sensor problem. Based on your repair experience and cost breakdown, replacing the sensor seems like the logical first step. I will schedule a diagnostic test and have them focus on the heat exchanger sensor area first. The coolant pressure test you mentioned makes sense too, better to catch any potential leaks now rather than after installing new parts. Those cost estimates help set realistic expectations for the repair. The tip about checking for coolant leaks near the sensor connection is particularly useful. Will make sure they inspect that during diagnostics. Hopefully this ends up being just a straightforward sensor replacement rather than the more complex AC control unit issue you encountered. Just to close the loop, I will report back once I get the diagnostic results and know more about whether its truly the temperature sensor causing these engine cooling system problems. Really appreciate the detailed troubleshooting guidance.