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Ok the problem here is the defrost system has failed, You will need to take the food out of the freezer and put it in a ice chest, next unplug the unit and take the shelf's out of the freezer. Remove the lower panel on the inside rear of the freezer, Once the panel is out you will see the evaporator (looks like a radiator on a car), this is going to be caked with frost/ice, thaw the ice/frost with a hair dryer. Once you have done this you will see the defrost heater looped below the evaporator. Test the heater with a ohm meter, If it reads open then it will need to be replaced.
If the heater tests good (reads continuity) then the defrost thermostat/sensor will need to be replaced. This is located clipped to the top corner of the evaporator.
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