A few possible things, but lets do a rough review of how the a/c works.

Just like your refridgerator, the a/c works by pumping a coolant or refridgerant (liquid) through high pressure to low pressure which lowers its temperature. Air is passed over this part of the plumbing (the evaporator, located in the dash with the fan) and it cools the air. The pump in your home fridge is an electric one, while the one in your car, called the compressor, is driven by a belt underneath the power-steering pump at the bottom driver's side front of the engine.

So this leaves to major things to go wrong:

1. Low or out of refridgerant. You may need to recharge the lines with freon, or that R-134a, or R-whatever stuff they use these days. If you are not so lucky, you may have a leak in the lines which means they all must be checked for leaks and replaced the ones with leaks.

2. If God hates you because you didn't file your taxes on time, your compressor might be out.


Time for self-diagnosis: Find the refridgerant line running from the compressor to the front of the car to the thing that looks like a mini-radiator. On the other end of this small radiator the line should continue to a small canister the size of a soda can. The fitting on top of this can should let the line run in-line, and there should be a small window on it. Run the car and the a/c on. In this window you should be able to see any air bubbles in the line, the more bubbles you have, the lower you are. If you see no movement at all and everything seems still, you probably have no refridgerant at all.

switch check: Run the car with the hood up. When you turn on the a/c knob, you should hear a small click from the switch, and listen to the engine as the engine speed should dip down slightly for a moment and the compressor should be turning. When a/c is off, the middle of the compressor pulley should be still, and spinning when the a/c is on. If this is not functioning correctly, the switch or electronics to turn it on are bad.

If you do seem to have refridgerant, and the switches are ok and the compressor is turning and you STILL get hot air, then you need a compressor change/rebuild like me!

Mine comes back tomorrow!

Although, lengthy, I hope i helped.