Dead AC in Audi TT, Compressor vs Relay Mystery

I experienced nearly identical symptoms on my 2012 Audi S4 last summer. The A/C compressor would not engage at all, similar to your situation. After diagnosis, the issue turned out to be a faulty relay switch and a bad pressure sensor, not the A/C compressor itself. The pressure sensor was preventing the system from engaging as a safety measure. When these components fail, the control module disables the entire air conditioning system to prevent damage to the A/C compressor. The repair involved replacing both the relay switch and pressure sensor. These parts are relatively accessible in the engine bay. This was significantly less serious than a complete compressor failure, which would have been much more labor-intensive. I suggest having a proper diagnostic test done first. The stored error code should help pinpoint whether its the relay switch, pressure sensor, or potentially something else in the control circuit. These components commonly fail around the 150-160k km mark on these engines. The belt tensioner should also be inspected while checking the A/C system, as it can affect compressor operation if worn. Getting ahead of these issues prevents more expensive repairs later.

I had a similar issue last year on my TT. So relieved when I read your response about the AC relay and pressure switch being the culprit rather than a complete compressor failure. The symptoms match exactly what I experienced. Can you remember roughly what you paid for the repair? In my case, I also noticed the refrigerant leak needed fixing a few months after the initial repair. Just wondering if you had any follow-up issues with your A/C system?

Glad I could help with the diagnosis. After taking my S4 to the mechanic, the A/C compressor shut off issue was completely resolved. The main problem turned out to be an undervoltage condition in the electrical system, which was causing the compressor to disengage. The total repair came to 240 Euro and the air conditioning has worked flawlessly since then. Unlike your case, I did not experience any refrigerant leaks afterwards. The pressure switch readings have remained stable, indicating the system is maintaining proper pressure. The A/C compressor cycles normally now and provides consistent cooling. If youre noticing refrigerant leaks, its worth having the system pressure tested. Low refrigerant levels can trigger the pressure sensor to shut down the system as a protective measure. Regular maintenance checks can help catch these issues before they lead to more serious problems with the air conditioning components.

Thanks for the input. Just wanted to close the loop on what happened with my TT. I took it to a new mechanic who actually specializes in A/C systems. The pressure sensor and electrical connections were indeed the main culprits. The total repair came to 280 Euro including a system pressure test and new refrigerant. They also noticed the belt tensioner was showing signs of wear, so I had that replaced while they were in there. Better safe than sorry with these components. The A/C compressor is working perfectly now, cold air blows strong and the system engages immediately when I press the button. Such a relief it wasnt a major compressor failure. The whole repair process took about 3 hours and the new mechanic showed me exactly what was wrong with the old parts. Really appreciate all the advice here. It helped me avoid getting oversold on unnecessary repairs and gave me confidence to find a more trustworthy mechanic.