RS4 AC System Failure: Compressor Not Engaging

As someone with moderate experience working on Audis, I had a very similar issue with my 2012 S4 last summer. The symptoms matched exactly what you describe, no compressor engagement and AC not cooling. After diagnosing, it turned out the AC relay and pressure switch were causing an undervoltage condition that prevented the compressor from engaging properly. The pressure sensor was also showing incorrect readings. The workshop found that the electrical system wasnt providing enough power to activate the AC components. The repair involved: Testing the AC pressure switch, Replacing faulty AC relay, Installing new pressure sensor, Recharging the AC system Total cost was 240 Euro for parts and labor. The system has worked flawlessly since the repair. Before jumping to conclusions about the compressor, it would help to know: What year is your RS4? Have you checked the belt tensioner condition? Are there any other electrical issues? What specific fault code is stored? The air conditioning problems could be electrical rather than mechanical. Getting these details would help narrow down if youre facing the same underlying issue I encountered.

Thanks for the feedback. After testing further, the stored fault code is P0533 AC Pressure Sensor Circuit High Input. Just checked and the AC relay appears to be working fine when tested. Honestly starting to suspect a refrigerant leak in the A/C system since the last service at 85000 KM showed everything was working normally. Will have the pressure switch and sensor connections double checked before assuming its a compressor issue. Hoping its just a minor electrical problem like in your case, but want to be prepared if its something bigger.

Good that you checked the AC relay, ruling that out helps narrow things down. The P0533 code you mentioned often points to issues with the pressure sensor circuit, but in my experience with these V8s, a refrigerant leak can trigger similar symptoms. When my S4 had comparable issues, the pressure sensor was showing incorrect readings because of low refrigerant levels caused by a slow leak. The system needs proper pressure to engage the compressor, so even a small refrigerant leak can cause these symptoms. Before replacing parts, I recommend: Having the refrigerant level checked with proper gauges, Inspecting all AC lines and connections for leaks, Testing the pressure sensor electrical connections, Verifying the pressure switch operation My repair ended up being simpler than initially feared, despite similar symptoms. The leak was found at a connection point, which was much cheaper to fix than replacing major components. The fact that it was working fine at your last service suggests a developing issue rather than sudden component failure. A pressure sensor costs around 85 Euro, but start with leak testing since thats the most common root cause with these symptoms.

Thanks for sharing those details. Based on the fault code P0533 and knowing the system was working at my last service, Im definitely going to get the refrigerant levels and potential leaks checked first before replacing any parts. The air conditioning pressure sensor connection issues youve described make a lot of sense given my symptoms. Ill have them inspect all the AC lines and fittings for leaks during the diagnostic check. Really hoping to avoid a full compressor replacement if its just a minor leak or sensor problem. Will get it into a shop next week to have the pressure switch tested and refrigerant system evaluated. The belt tensioner and other components seem fine from my inspection, but better to have everything checked properly. At least now I know what to tell them to look for specifically. Good to hear your repair turned out to be relatively straightforward. Fingers crossed mine is similar and just needs a simple fix rather than major AC system work. Will update once I know more about what they find.