Ice buildup on evaporator. I used a multimeter and tested defrost heater, defrost temperature sensor, and fusible link. Heater and link had continuity, and resistance OK in heater. Temp sensor had continuity, but did not indicate correct resistance readings. Heater did not come on in forced defrost mode. Ordered new temperature sensor which arrived promptly 3 days later! Installed it in a few minutes and everything worked normally. Beer got cold, I drank one. Very impressed with the website help and the service I received.