I own a shop and we do about 20 a week. Here's the deal if a GE doesn't defrost. First visually inspect the defrost sensor, it should not be bulged. If it is test it then relplace it. If it's good, test the element. If it's bad replace it, if it's good the we replace the thermistor and the heater comes on every time. That the little white thing on a silver clip attached to the copper return tube on the evaporator.