The ACs in our cars today use two different types of control valves one is electronic, and the other is mechanical. The mechanical valve uses what's called a precision diaphragm which senses the pressure on the lower side. So, when the cabin feels warm, the temperature of the evaporator increases which in turn causes an increase in the low side pressure, consequently collapsing the diaphragm. Then the port opens to vent the building pressure into the side of the installed compressor head, which then decreases the pressure in the housing and increases the number of piston strokes which leads to more refrigerant flowing through the system.
There is no sure way to know if the problem with your AC is the control valve without opening up the system and examining it carefully. However, in most cases if your AC seizes to cool properly it's the control valve that needs to be replaced. It is only once the control valve is replaced with the exact same one that the system can function optimally. That said a lack of cooling can also be attributed to a malfunctioning compressor. That is why the AC system needs to be examined by a professional.
The best way to buy the right replacement control valve for your AC is to match the part number of the existing one to the ones listed above. We currently sell control valves from top OEM manufacturers like WABCO, ELRING, and VAICO among many others. If you need help finding, the right valves do not hesitate to get in touch with us for assistance.