There are a number of reasons for replacing an expansion valve:
1. Mechanical failure of the valve: possible seizure of the valve stem carriage; worn valve seat and letting by; defective or wrong sized valve orifice cartridge.
2. Partial loss of the thermal charge: insufficient pressure exerted by the power head in response to temperature changes, resulting in an erratic valve operation.
3. Complete loss of the thermal charge from the power head: the spring pressure will close the valve and refrigerant flow to the evaporator will cease.
4. An excessive pressure drop across the evaporator is discovered: an external equalizing type has to be substituted.