The expansion valve is the gatekeeper of the cooling cycle. Located at the firewall between the engine bay and passenger cabin, it controls the flow of high-pressure liquid refrigerant into the low-pressure evaporator. This sudden drop in pressure causes the refrigerant to boil and turn cold, absorbing heat from the cabin.
If the expansion valve sticks closed, refrigerant cannot flow, and the A/C will blow warm air. If it sticks open, too much liquid enters the evaporator, which can freeze the core fins or send liquid refrigerant back to the compressor, locking it up.