Audi A4 AC Failure: Compressor and Switch Issues Fix

As someone with some experience working on Audis, I had a very similar issue with my 2005 Audi A6 last summer. The A/C system completely failed, and the A/C compressor wouldn't engage either. After checking with my trusted workshop, they found the root cause was undervoltage in the vehicle, which was affecting the AC relay and other electrical components. The complete diagnosis and repair, including a new relay switch and system check, cost me 240 Euro. The workshop also performed a thorough check of the A/C system to ensure there weren't any refrigerant leaks or other complications. The fix worked perfectly, and the cooling has been working flawlessly since then. Before suggesting specific solutions, it would help to know: Have you noticed any other electrical issues? Does the fan still work when you turn on the A/C? What error code is showing specifically? Have you had any recent battery or alternator issues? These details would help determine if your issue matches what I experienced with my A6. The symptoms sound very similar, but with these cars, its important to rule out other potential causes before replacing parts.

My mileage is 139182KM and the last service was done 3 months ago. Thanks for the detailed response. Yes, the fan does work when the A/C is turned on, but I noticed that the pressure sensor light flickers occasionally on the dashboard. No other electrical issues so far, and the battery was replaced last year. The error code showing is P0645 which points to the A/C clutch relay circuit. Im wondering if its worth checking the pressure switch first since its typically cheaper than replacing the A/C compressor or dealing with larger HVAC system repairs. The relay switch diagnostic would be helpful too, but I want to be methodical about the troubleshooting. Did your repair also involve testing the pressure in the system before replacing components?

Having gone through this exact situation with my A6, I can confirm that pressure testing is absolutely crucial. When my workshop did the diagnostic, they first checked the pressure levels before touching any components. The P0645 code you're seeing is exactly what I dealt with, and it does indeed relate to the AC relay circuit. The flickering pressure sensor light you mentioned is particularly interesting because this wasnt present in my case. While my issue was resolved with a relay switch replacement, your symptoms suggest it would be smart to check both the pressure switch and refrigerant levels first. A failing pressure switch can cause the compressor clutch to disengage, which matches your symptoms. The fact that your fan works when the AC is on rules out some electrical issues I initially suspected. Given that your battery is relatively new, I would focus on: 1. Testing system pressure and refrigerant levels 2. Checking the AC relay 3. Inspecting the pressure switch 4. Testing the belt tensioner that drives the AC compressor My repair started with pressure testing which cost about 80 Euro, but it saved money by pinpointing the exact issue. The flickering pressure sensor light suggests you might be dealing with a different root cause than I did, possibly a combination of low refrigerant and a failing pressure switch. Would strongly recommend getting these specific components checked before any major HVAC system repairs.

Thanks for all this helpful info. The pressure sensor light flickering definitely seems like it could be the key here. I think Ill start with the pressure testing to rule out refrigerant issues, then move on to checking the pressure switch and relay switch if the pressure test comes back normal. The belt tensioner check is also a good call, I hadnt considered that could affect the A/C compressor engagement. My mechanic already quoted me 90 Euro for the full diagnostic including pressure testing, which seems reasonable based on what you paid. Ill have them focus on these specific components rather than jumping straight to major repairs or A/C compressor replacement. P0645 matching your experience gives me hope this wont be a huge repair. Ill update once I get the pressure test results and know more about which components actually need replacing. Really appreciate you walking through your similar experience in detail.