Have the evaporator exposed. It is again frosting over after only two days. Very light coating. It appears to me that the defrost heater and thermodisc are wired together. Red and black from the control board, then split into three, red, black and orange. Do I test at the plug for continuity? If so, I have continuity when they are cold. Does this mean they're good? Could the temp sensor on the evaporator be bad and causing this?
For the bi metal, this is a overtemperature thermostat and should read closed at all times under 155 degrees. This most likely will not be the issue. Test the thermistor that is located on the evaporator and make sure that while sitting in ice it reads approximately 42.2k ohms at 0 degrees 14.4K ohms at 37 degrees. This will most likely be the cause but if heater, thermistor and bimetal check ok then the issue will be the control board.
There is also a temp sensor on the condenser line. Could it be bad or contributing to this as well? Once defrosted it cools right down and works great.
There is not a temperature sensor on the condenser line but there is a sensor on the evaporator located in the back of the freezer compartment that if it is faulty will cause the unit to not defrost.