The AC expansion valve controls the amount of the cooling agent flowing into the evaporator. If the AC system is not working efficiently, the first thing to check is the valve. If it is clogged, not enough of the cooling agent flows into the evaporator. Consequently, the AC is not working efficiently and the compressor wears faster. If the valve lets too much of the agent through, it accumulates in the evaporator, freezes and affects the operation of the entire system.