When it gets warm make sure the belt is tight. You also need to add freon at the beginning of every
warm season. You can also make sure the the blower fuse is good.
AC units need routine inspection of the various hoses and occasional flushes and refills of the refrigerant. The drive belt for the compressor needs to be replaced on a regular basis as well; on most cars that belt also turns some other accessory like the alternator. They should only be worked on by a professional; take it to your dealer.
Cleaning the condensing unit coils, checking amp draw of compressor, oiling the fan motors, checking that belts, checking operating pressures, temperatures against the manufacturers specifications. Check is the coolant level
To ensure proper working order of your car's air conditioning, regularly check the refrigerant levels. Low refrigerant levels should be refilled and and the car checked for leaks.
