As the compressor compresses refrigerant, the high pressure raises the temperature of the refrigerant, but as it reaches the evaporator core, it passes through an expansion valve. The expansion valve could easily be called a depressurizing valve as well, as it rapidly depressurized the refrigerant, causing the temperature to drop dramatically. The refrigerant in its cold, gaseous state passes through the evaporator core, causing the metallic core to become extremely cold. The cold evaporator coil is placed in the path of airflow of the blower motor, which causes the air blowing through the evaporator and into the vehicle to become cold as well. Usually the main cause of faulty vaporator is leakage at tube joints due to aging.