To understand how the expansion valve works in a modern AC, you must realize that temperature plus pressure have a direct relationship to one another. Increasing the pressure of a refrigerant also increases its temperature, packing its related molecules more tightly together. This dual increase is exactly what happens whenever the refrigerant passes through the compressor. The pressure plus temperature relationship necessarily works in the opposite direction as well, then by reducing the refrigerant’s pressure inside, you can cool the refrigerant off effectively. The expansion valve’s main purpose is to restrict the flow of refrigerant into the evaporator coil. Without the expansion valve in place, the refrigerant would rush into the evaporator coil without losing any of its required pressure. The expansion valve only permits a small space of the refrigerant to enter the evaporator coil. As the refrigerant enters, it finds itself able to spread out in the large, empty evaporator coil. This expansion causes its pressure to decrease abruptly plus the internal temperature drops along with it. As far as air conditioning components go, expansion valves honestly tend to be tough. As a result, they experience fewer issues than more delicate parts of a HVAC method such as a compressor, then expansion valves have to withstand immense amounts of internal pressure from the liquid line side so they have to be built tough, but that being said, over time, this pressure can cause the expansion valve to break down plus if it finally fails, it will not be capable of limiting the flow of refrigerant plus your AC method will not be able to cool as efficiently.