Good day,
You will need an ohm meter to check the parts.
Your defrost system has failed.
It is made up of 3 components, 2 of which are located behind the rear panel in the freezer compartment.
Remove the rear panel. There is a cooling coil, that has a calrod heating element that runs along it's bottom. As well there is a defrost thermostat ( a small round 1" disk) which is located on the top of the same coil. Both should be checked with an ohm meter to test for continuity. The heater should read no more than 30 ohms, the thermostat ( IT MUST STILL BE COLD BELOW 30 DEGREES) should read zero ohms. Note: The heater rarely fails, the thermostat has a high failure rate.

The third item is a timer, or a solid state adaptive defrost control. Depending on age, and model, it is located , behind the kick plate on the bottom front.
Or, in the fresh food compartment on the roof.
If it is a timer, you can turn the timer shaft when the machine is running until the machine shuts off. With it still turned on, wait 40 minutes.
If the machine turns itself back on, the timer is O.K. if not, the timer is bad, and needs replaced.
If the Main heater checks O.K. and it has a solid state control, then replace the thermostat in the freezer compartment, and the solid state board.

Below is an excellant tutorial on defrost systems.

http://www.appliance411.com/faq/howdefrostworks.shtml
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