Fixing the AC
1. Check to see if all the R-134a leaks out (meaning there’s nothing in the loop to carry away heat).
Leaks are easy to spot but not easy to fix without pulling things apart. Most auto-supply stores carry a fluorescent dye that can be added to the system to check for leaks, and it will have instructions for use on the can. If there’s a bad enough leak, the system will have no pressure in it at all. Find the low-side valve and with a gauge check the PSI level. Do not use anything else to poke in the valve to see if it will come out, this is illegal. It’s called venting.
2. Make sure the compressor is turning.
Start the car, turn on the AC and look under the hood. The AC compressor is generally a pump-like thing off to one side with large rubber and steel hoses going to it. It will not have a filler cap on it, but will often have one or two things that look like the valve stems on a bike tire. The pulley on the front of the compressor exists as an outer pulley and an inner hub which turns when an electric clutch is engaged.
If the AC is on and the blower is on, but the center of the pulley is not turning, then the compressor’s clutch is not engaging. This could be a bad fuse, a wiring problem, a broken AC switch in your dash, or the system could be low on refrigerant (most systems have a low-pressure safety cutout that will disable the compressor if there isn’t enough refrigerant in the system).
3. Look for other things that can go wrong. The other sorts of problems that the AC might be experiencing include: bad switches, bad fuses, broken wires, broken fan belt (preventing the pump from turning), or seal failure inside the compressor.
4. Feel for any cooling at all. If the system cools, but not much, it could just be low pressure and you can top up the refrigerant. Most auto-supply stores will have a kit to refill a system, complete with instructions.
Do not overfill! Adding more than the recommended amount of refrigerant will not improve performance but actually will decrease performance. In fact, the more expensive automated equipment found at nicer shops actually monitors cooling performance in real-time as it adds refrigerant. When the performance begins to decrease it removes refrigerant until the performance peaks again.