If your car's air-conditioning system blows only warm air on hot days, it's probably because it is low on refrigerant, and the most likely cause is a leak somewhere in the system.
Though many vehicle owners think they need to recharge or "top off" their air-conditioning system with refrigerant on a regular basis, such as annually, that's not the case. If the air conditioning stops working (but the fan continues to blow warm air), a leak in a hose, connector or other part of the system is probably the culprit. During the winter, you may notice that the defroster isn't clearing fogged up windows, and that also is a sign you have a leak. Most defrosters engage the air conditioning to dehumidify the air.
If there is a leak, the air-conditioning system on most modern vehicles is designed to shut down once the refrigerant level drops low enough in order to prevent damage to the air-conditioning compressor. Note that we are talking about "refrigerant," not Freon, the brand name for a type of refrigerant that is no longer manufactured in the U.S. because it can damage the ozone layer. Contemporary vehicles use a refrigerant known as R-134.
The EPA provides helpful information for consumers about whether they should have their air conditioner topped off with refrigerant or evacuated and recharged, found here . The EPA notes that a repair shop (or a do-it-yourselfer) can't tell precisely how much refrigerant is in the system without the proper equipment — gauges to measure how much pressure is in the system — so how much they add while topping off the system is largely a judgment call.
By performing a complete evacuation and recharge (a more expensive procedure and one that should be done by a professional), the EPA advises, the repair shop will be able to test the complete system and recharge it with the precise amount of refrigerant recommended by the vehicle manufacturer. The EPA adds that usually there is no reason to clean the system unless it is opened up, such as to check for leaks or other problems.
When is it time to add or replace the refrigerant? You could be proactive and have it done before you experience problems, but you shouldn't need that more often than every few years at the most. If your air conditioning is losing its potency, even after topping it off, then you probably have a leak. But if you stay as a cool as a cucumber on the hottest days, you could just leave well enough alone. The air conditioner obviously has enough refrigerant.