An automotive air-conditioning system requires many components to function properly. When a failed component is replaced, typically, the system must be recharged with refrigerant. When a mechanic attempts to refill the system with refrigerant, most commonly R134a or R12, and the system will not accept a full charge, there can be several causes.
The air-conditioning system in a vehicle has a high- and a low-pressure side. Both sides require specific pressure ranges: the low-pressure side requires 20 to 25 psi; and the high side requires 200 to 225 psi with R12, and 225 to 250 psi with R134a. When there is a clog in the air-conditioning system, these pressures are reached without the charging station reaching the proper refrigerant level; at this point, the charging station will not allow any more refrigerant to enter the system.