Freezer won't defrost. It is not the defrost heater. How do I diagnose the other things it could be?

Hi. If the heater is not the faulty part, you could have a bad defrost sensor, bi-metal, or control board. The sensor is tested for continuity. Depending on the temperature that the sensor is at you will get a different ohm reading. The chart is on page 49 of the service manual I have attached for you. The bi-metal can just be checked for continuity. It should be closed. If the sensor and bi -metal both test good, then the only thing left is the control. You will need to replace the control if the sensor, heater, and bi- metal test good.