Audi S4 AC System Dead: Compressor vs Relay Issues

Having experienced similar AC issues with my 2014 Audi S5, I can relate to this problem. The symptoms you describe match what happened in my case, where the air conditioning system shut down completely. The initial diagnostic revealed an undervoltage condition affecting the A/C compressor circuit. While it might seem like a compressor failure, the root cause was actually a faulty AC relay combined with a refrigerant leak from a deteriorated seal. Here are the typical checkpoints for this issue: AC relay function test, System pressure check, Electrical voltage measurements at the compressor, Belt tensioner inspection In my case, replacing just the compressor would not have solved the underlying issue. The complete fix required: 1. New AC relay 2. Refrigerant system evacuation and recharge 3. Replacement of worn seals 4. System pressure test This was a moderately serious repair that required specialized equipment for proper diagnosis and repair. Simple visual inspection wasnt enough to identify the full scope of the problem. Worth noting that ignoring these symptoms could lead to more extensive damage to the A/C compressor if you continue trying to operate the system.

Thanks for the detailed information about the AC problems. I had very similar issues with my Audi S4 last summer. The A/C compressor also shut down completely and needed professional diagnosis. After some back and forth with the shop, they found a cracked pressure switch and a minor refrigerant leak in the system. The whole repair process took about 2 days. Just wondering, do you remember roughly how much you ended up paying for the full repair? And has the air conditioning system been working properly since the fix? My repair bill seemed quite steep but the AC has been running perfectly for the past months.

Good to hear your A/C system troubles were resolved. After taking it to my trusted mechanic, the air conditioning has been working flawlessly. The repair focused on fixing the undervoltage issue affecting the A/C compressor shut off mechanism, which was causing the system failures. The total cost came to 240 Euro, which covered the complete diagnosis, parts, and labor. This actually turned out to be quite reasonable considering the complexity of modern vehicle AC systems and the specialized equipment needed for proper testing. The pressure switch and relay switch validation were essential parts of the diagnostic process. Once the electrical issues were sorted, the A/C compressor engaged properly and the entire system returned to normal operation. Its been perfectly reliable through all weather conditions since the repair. Looking back, the thorough diagnostic approach helped avoid unnecessary parts replacement and kept the costs manageable. Regular maintenance of the air conditioning system since then has helped prevent any recurrence of the problems.

I took my S4 to a different shop this time, and Im actually glad I did. After checking the AC relay and running a full system diagnostic, they found the actual culprit, a failed pressure sensor and a slow refrigerant leak in the A/C system. The repair took about 4 hours total. They replaced the faulty sensor, fixed the leak, and did a complete system recharge. Cost me 380 Euro all in, which feels fair given the work involved. The AC has been working perfectly for the last 3 months now, good cold air and no more error codes. Must say the shop did a thorough job checking everything, way better than my previous experiences. They even showed me the worn parts and explained what caused the failure. Think Ill stick with these guys for future AC issues. Its nice to finally have the cooling working properly again without worrying about compressor shutdown every time I start the car.