Low refrigerant
The AC system is sealed, so low refrigerant usually means there is a leak somewhere. Hoses, seals, and connections can wear over time. Simply adding refrigerant may cool the car for a while, but the leak will remain unless it is found and fixed.
Other common causes
The compressor may not be engaging because of a failed clutch, a blown fuse, a bad relay, or a pressure problem. A blocked or damaged condenser, which sits near the front of the car, can keep the system from shedding heat.
A faulty blend door actuator can also cause warm air, because it controls whether air passes over the heater or the cold evaporator.
Quick things to check
Make sure the temperature is set to cold and the AC button is on. Listen for the compressor clicking on when you turn the AC on. If airflow is weak, a dirty cabin air filter may be part of the problem and is often easy to replace.
Avoid driving long with strange noises from the compressor, since a failing one can spread debris through the system.
Getting it diagnosed
A technician can check system pressures, look for leaks, and test electrical parts to find the actual cause. Having this done before the hottest weather arrives helps you avoid the rush when the AC is needed most.
