Audi S4 AC Failure: Compressor Warning Signs & Solutions

Having decent experience with auto AC repair, I faced an identical issue with my 2012 Audi S5. The symptoms were exactly the same, complete AC failure, warning light, and no engagement. Initially suspected low auto refrigerant, but the diagnostic showed a failed AC control unit. After checking multiple potential causes, the repair shop confirmed the control unit was the culprit. The fix required a complete replacement of the control unit, costing 980 Euro. The repair process took about 4-5 hours since they needed to: Run complete system diagnostics, Remove the old control unit, Install and program the new unit, Test the automotive cooling system thoroughly Before proceeding with any repairs on your S4, could you share: Any unusual sounds before the failure? Did the AC performance gradually decrease or fail suddenly? Are you getting cold air at all, even briefly? Have you noticed any refrigerant leaks under the car? This information would help determine if your issue matches what I experienced with the control unit failure or if it might be something else in the air conditioning repair system.

Last service was at 168000 KM, and I am not getting any unusual sounds from the car air conditioning system. The failure was quite sudden, worked fine one day, completely dead the next. No signs of gradual decline in cooling performance. Have not spotted any leaks under the car, and the AC compressor is not engaging at all when the button is pressed. The temperature stays at ambient no matter what setting I choose. I will have it checked for potential refrigerant issues first since its the most common auto AC repair cause, but your control unit experience is really helpful to know about. Going to have it properly diagnosed next week to be sure.

Thanks for the update on your Audi S4 AC issue. Based on your description, your symptoms align even more closely with my control unit failure experience than I initially thought. The sudden complete failure without warning signs is particularly telling, it matches exactly what happened with my S5. When my vehicle HVAC system failed, I also had zero compressor engagement and no cooling whatsoever. The fact that you are not hearing any unusual sounds and see no refrigerant leaks further suggests a control unit problem rather than a mechanical compressor failure. While checking the auto refrigerant level is a sensible first step, I would bet your cooling system issues stem from the control unit. In my case, they first suspected low refrigerant too, but the diagnostics revealed the control unit had completely failed. If it turns out to be the same issue, prepare for a similar repair cost around 900-1000 Euro. The good news is that once replaced, the air conditioning repair resolved all issues and the system has worked flawlessly since then. Would definitely recommend having them check the control unit specifically during diagnostics, given how similar your symptoms are to what I experienced.

Thanks for the input. The similarities between my S4 issue and your control unit experience are really striking. The sudden complete failure of the vehicle HVAC system without any warning signs definitely seems to point in that direction rather than a gradual refrigerant leak or mechanical problem. Will still have them check the auto refrigerant levels first since its a quick diagnostic, but I am now leaning towards the control unit being the culprit. The fact that my air conditioning repair symptoms match so precisely, no compressor engagement, no cooling, and sudden failure, makes me think this could save some diagnostic time. Have scheduled the automotive cooling system check for next week and will make sure they specifically examine the control unit. Good to know about the potential 900-1000 Euro cost range so I can plan accordingly. Will update once I get a proper diagnosis and repair completed.