So you have turned on your car’s air conditioning and the air that comes out is not cold. If you don’t feel any air coming out of the vents, but hear the fan, the fresh air intake filter (more commonly called the cabin filter.) may be restricted or partially blocked. If you park near a tree or park outside, there could be a rodent of some kind who has made a comfy little home for themselves in your vents. If that is the case, the nest can be removed and the filter replaced. The vents can even be sanitized if needed.
What if you have air out of the vents, but it is not cold? There could be any of a number of things going on. There could be a blown fuse or relay, there could be a leak in the system, there could be a bad component or the system may simply need to be recharged. It could even be a broken drive belt. The system will seep tiny amounts of refrigerant over time, very slow leaks are not that uncommon. There are a couple of types of refrigerants that are common. They have often been called Freon. There are new ones on the horizon as options are being tested that are more environmentally sound.
A visual inspection is performed prior to service to look for obvious problems. If all looks ok, then the technician will proceed with a recharge. When the system is being charged (or serviced) it is evacuated and checked for leaks under vacuum. The amount of Freon removed from the system is checked and documented. If no vacuum leaks are found, the Freon will be added to specifications and the system checked for leaks under pressure. The system will be checked to make sure that it is operating as designed and the vent temperature is documented. This way you know just how cool you are!