Thanks, the evaporator coil was frozen over, so I used a blow dryer and melted all the ice. I pulled the plug on the heating element which has the defrost thermostat attached to it and checked the resistance.
For the heating element, it showed about 20 ohms. The defrost thermostat showed 240 ohms, as specified in the wiring diagram.
My thought is the adaptive defrost board and/or the temp control, the next things in the electrical path.
Would prefer not to spend $80 for a defrost board unless it is necessary. Any diagnostic for that?
When I turn the freezer temp control to off, compressor and all fans go off, so at least that much of the temp control works.
The unit has always frosted over food in the fridge for a long time, but at least it got cold.
Maybe replacing both parts is the safe thing to do?