Things that can cause a heat pump to freeze up are:Low refrigerant level or refrigerant leaks. …A bad heat pump defrost control board. …A bad heat pump thermostat or thermistor. …A bad or sticking reversing valve. …A slow or dragging outdoor or indoor fan motor. …Any kind of restricted airflow on the outdoor unit like maybe a garbage bag has blown up against the outside of the unit. …More items…