Moisture in the System
If moisture enters the air conditioning system, it freezes at the system's metering device. Notice the frost at the expansion valve's port above, where the refrigerant is metered into the evaporator core. (TXV) thermostatic expansion valve air conditioning systems contain a receiver drier that cleans and absorbs moisture from the refrigerant. If the drier becomes saturated with moisture, it releases it at the hottest times of the day. This moisture freezes in the expansion valve and deprives the evaporator of refrigerant. This is why the customer was able to drive the vehicle for five or ten minutes before the valve froze, and A/C performance was reduced.