I checked the thermostat by cutting the wires to it and connecting them with a twist connector then forcing a defrost cycle, no defrost, also placed the thermostat clip down in a very small plastic bowl with 1/2" of water and froze it, tested OK. I checked the heater with an ohm meter, 30 ohms - within range, also checked voltage to heater during forced defrost - only 20 volts. Heater OK. Replaced the control board and the fixed the problem. Freezer now defrosts as it should.