Answer Hugh, if the evaporator is blocked up with ice, the defrost thermostat would need to be tested for continuity when the power supply cord is unplugged. If it does not have continuity, it has failed and would need to be replaced. If it does have continuity, the defrost heaters would need to be tested for resistance. If they do not have resistance, or reads infinite, the heater in question would need to be replaced. If the heaters do have resistance, the defrost timer would be the cause of failure. Read More... Answered by AppliancePartsPros.com | Wednesday, August 23, 2023