In a perfect world the air conditioning system in your car should not lose freon (refrigerant). It is a closed system and the freon does not get used up. If your AC is not blowing cold and you need freon added, there is a leak somewhere. Repairing the AC and getting it to blow cold again could be a multi step process.
The first thing we will check is to make sure that the A/C compressor is turning on. If the compressor is not turning on there could be a few causes. Either there is an electrical issue (power supply), mechanical damage to the compressor or there is not enough freon in the system to activate the pressure switches to turn the compressor on. If the compressor is turning on, then we will perform a pressure test to determine if the compressor is putting out the correct amount of pressure for the system.
If the compressor is working properly, but there pressures are not correct due to a low freon charge, we will perform an evacuation of the system. This removes any freon left in the system and measures the amount while placing the system in a vacuum. If the vacuum pressure drops, it is an indication that there is a leak somewhere. If we cannot determine where the leak is we will recharge the system with freon according to the factory specification and we will inject a dye into the system. This dye will help us to see where the freon is leaking. We ask the customer to return a few weeks later so we can look at the system under a UV light to see where the dye is coming from and will allow us to determine where the leak is coming from and what needs to be replaced.