Audi A4 AC Compressor Failure: Diagnosis & Fix Guide

Having worked on cars for a few years now, I encountered a similar issue with my 2005 Audi A6 last summer. The auto AC repair turned out to be more complex than just a compressor failure. The vehicle HVAC system showed similar symptoms, no compressor engagement and warning lights. Initially, the workshop suspected the AC compressor, but diagnostic testing revealed a faulty AC control unit. The control unit manages the entire AC system, including compressor engagement, and when it fails, it can mimic compressor problems. The repair cost me 980 Euro, which included: New control unit, System diagnosis, Refrigerant recovery and recharge, Labor costs Before jumping to conclusions about the compressor, could you share: Are you getting cold air at all, even intermittently? Does the blower motor work on all speeds? What specific error codes are you seeing? Have you noticed any unusual noises when trying to engage the AC? This would help determine if your issue matches what I experienced with the control unit or if its actually a compressor problem. The AC unit issues can be tricky to diagnose without proper testing, as multiple components can cause similar symptoms.

Thanks for sharing your knowledge. Last service was 2 months ago at 160180 KM. To answer your questions: the blower works fine on all speeds but there is no cold air at all. The error code P0645 points to the AC compressor circuit. When starting the car, I hear a clicking sound from the compressor area, then nothing happens. I checked for auto refrigerant leaks with a UV light but found no obvious signs. The system pressure seems normal according to the gauges at idle, suggesting the refrigerant level is okay. This makes me think the AC compressor itself might be the culprit rather than the control unit you mentioned. Would it make sense to check the compressor clutch first before replacing the entire unit? The clicking noise makes me wonder if its just the clutch mechanism failing.

Thanks for those extra details. Like you, I also heard that distinct clicking sound in my A6 before the complete failure, but I initially went down the wrong path checking the clutch. The clicking followed by no engagement often indicates an electrical issue in the compressor circuit, which matches your P0645 code. In my case, even though the clutch seemed like the obvious culprit, the car air conditioning problem was actually caused by a worn compressor that was drawing too much current, causing the control module to shut it down as a protective measure. A few things worth checking before a full compressor replacement: Compressor clutch gap (should be 0.4-0.8mm), Voltage at the compressor clutch connector, Fuses and relays for the AC system However, based on your description and comparing it to my experience, I suspect youll end up needing a full compressor replacement. When I finally had mine done, it cost around 1200 Euro including: New AC compressor, Receiver drier, Expansion valve, System flush, Fresh refrigerant The automotive cooling system professionals recommended replacing these additional components to prevent contamination from the old compressor affecting the new one. Just replacing the clutch might work temporarily, but its often a band-aid solution when the core issue is compressor deterioration.

Thanks for the detailed response. You make good points about the electrical diagnosis. The more I think about it, the clicking sound followed by no engagement really does suggest a bigger issue than just the clutch. After looking at the repair costs you mentioned, I might as well do the complete AC compressor replacement along with the other components. Better to fix it properly than risk having ongoing vehicle HVAC problems. The fact that my symptoms match your experience with current draw issues makes me more confident about proceeding with the full repair. Called a few shops today and got quotes ranging from 1100-1400 Euro for the complete job including the receiver drier and expansion valve replacement. Going to schedule the repair next week since the summer heat is coming and driving without a working cooling system isnt fun. Really appreciate you sharing your experience, helped me avoid wasting time and money on partial fixes that probably wouldnt have solved the underlying problem. Will update once the repair is done to confirm if it was indeed the compressor causing all the trouble.