Refrigerator GE GSS22KGMDWW defrost failure

Question

The heater comes on but doesn't completely defrost the coils before quitting.
I replaced the motherboard last month. I replaced the thermostat when I replaced the heater three years ago.
I think the defrost thermostat goes directly to the motherboard on this model rather than being in series with the heater.
Heater resistance is about 28 ohms. At room temperature the heater thermostat is a short.
I replaced the ice maker, which I removed a couple of years ago when it quit working, thinking that maybe the change in cold air flow might have affected the defrosting.
The fan in the freezer is working. The freezer section works okay.