[artm]

Went on a three week vacation. Knowing my battery would likely be shot due to the vampire nature of the aftermarket head unit, I disconnected the positive lead on my battery before leaving.
No dead battery, but now I have no A/C. Fan is running, controls are lit, just no cold air.
Can hear a "ERRRRR" "ERRRRR" under the hood when the AC button is engaged.

Ideas?

2011 2.4 LTZ, climate control

 

[Drunken Elvis]

Sounds like a mechanical problem under the hood.
Guess.....the AC compressor has seized due to lake of use and the belt is now slipping on the compressor pulley.[this would be very loud]
or
the belt tensioner has seized in a position that does not apply much tension to the belt and the belt is too loose to turn the AC compressor.

I suggest you access the AC pulley and try to turn it slightly with the appropriate wrench/socket. Maybe you can free it up. Check the belt tension while you are there.