Your air conditioning system relies on a closed system that circulates refrigerant through a compressor, an evaporator, a drier and one or more heat exchangers to pull heat out of your car and dump it outside, leaving you 20 degrees or so cooler inside than out. O-rings and gaskets seal the joints in this system and keep the refrigerant inside. Your compressor moves refrigerant through the system, where it expands to a gas then returns to liquid to “condition” the air without being used up. Refrigerant gases are environmental hazards if released into the air, so we use special machines to evacuate them from your A/C system and return them after repairs. When your car sits unused, the tiny seals inside your climate-control system can shrink slightly and let refrigerant escape. Over time, you may lose just enough refrigerant that your vents blow coolish but not cold because you’re low on refrigerant. We’ve seen cars with just a few ounces of refrigerant short of a full load, and the customers notice poor A/C performance, especially when it’s really hot. Your A/C also contains lubricants and other additives for the compressor and other components and those escape, too, when you have a slow leak. Even a rock striking your evaporator can poke a hole in the system.