The expansion (orifice) tube is a plastic assembly containing a fixed diameter tube with a mesh filter screen at either end. The fixed diameter tube creates a restriction to the high pressure liquid refrigerant in the liquid line, metering the flow of the refrigerant to the evaporator as a low-pressure liquid. When the engine is turned OFF with the A/C system operating, the refrigerant in the system will flow from the high-pressure side of the expansion tube (orifice) to the low-pressure side until the pressure is equalized. This may be detected as a faint sound of liquid flowing (hissing) for 30 to 60 seconds and is a normal condition.

When system diagnostics indicate a restricted expansion tube, it may not be necessary to replace it. Metal chips, flakes or slivers found on the screen may be removed with compressed air and the expansion tube may be reused if: