Warm air blowing from the vents
AC takes longer than usual to cool the cabin
Weak airflow from the vents
AC clutch cycling on and off frequently
Compressor not engaging consistently
Frost or ice on AC lines
Hissing sound from the AC system
Visible oily residue around AC fittings or hoses
AC works better while driving than at idle
Cabin air is cool but not cold
Pressure readings are low when checked with gauges
AC system has not been serviced in a long time