A6 AC Failure: Heat Exchanger Sensor Diagnosis

Having experienced a similar issue with my 2006 A4 2.0T, I can share some relevant repair insights (medium experience with HVAC systems). The symptoms you describe match what I encountered, AC failure with dashboard warnings. However, in my case, the temperature sensor was fine, but the HVAC system control unit itself was defective. After initial diagnosis pointed to the sensor, my mechanic found the control unit was not properly communicating with the heat exchanger components. The repair involved: Complete diagnostic scan, Testing the temperature gauge and sensor connections, Replacing the control unit, Recalibrating the system, Checking for any coolant leaks Total cost was 980 Euro for parts and labor. The repair resolved all issues and the AC has worked flawlessly since. To provide more specific guidance, could you share: Any specific error codes showing up? Does the AC blow warm air or no air at all? Have you noticed any coolant level changes? When did the problem first appear, suddenly or gradually? Any unusual sounds from the HVAC system? This information would help determine if your issue matches what I experienced or if it might be something different.

Thanks for the detailed information about the control unit. Just had my 136193 KM service done last week, and I am now seeing similar warning lights. The AC blows warm air intermittently, and the error message started appearing right after. No strange sounds from the system yet. The sensor calibration process you mentioned seems logical, but I havent noticed any coolant leaks around the heat exchanger area. The engine cooling system appears to work normally, with temperature staying steady at operating range. Did your repair require any special tools for accessing the control unit? The part cost seems steep, wondering if there are any reliable aftermarket options worth considering.

Based on your timing with the recent service, this actually sounds very similar to what I dealt with. The intermittent warm air was exactly my experience before the complete failure occurred. Since you mention the 136k service, its worth noting that my sensor malfunction appeared shortly after routine maintenance too. The temperature gauge remained stable in my case as well, which initially made diagnosis tricky. Regarding special tools, the control unit replacement needed a diagnostic computer for proper sensor calibration and coding. I initially considered aftermarket parts to save money, but ended up going with OEM after reading about compatibility issues. The heat exchanger temperature sensor is pretty integral to the whole system working correctly. A few cost-saving suggestions from my experience: Get the temperature sensor tested separately before replacing the control unit, Check all fuses and connections first, Consider having an independent shop do the repair, but use OEM parts, Ask for the old control unit back to potentially claim warranty The aftermarket units I researched were about 40% cheaper but had mixed reviews about sensor calibration compatibility. My advice would be to avoid that route given the complexity of these systems.

Thanks everyone for the helpful responses. I think I'll start by having the temperature sensor tested independently as suggested, since that's a more manageable first step before considering the full control unit replacement. After hearing about the service correlation and similar symptoms, I'm more confident in the diagnosis. The stable cooling system readings but intermittent AC issues match what others experienced. I'll avoid the aftermarket parts route given the feedback about compatibility problems with the heat exchanger temperature sensors. Will book a diagnostic test this week and have them check all connections and fuses first. The 980 Euro repair cost is steep but seems worth it if it fully resolves the issue like it did in the other case. At least now I know what to expect and what questions to ask the shop. Will update once I get more definitive test results back.