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This can be caused by a couple of things.
The first would be a clogged defrost drain. In order to clear the defrost drain you would need to unplug the unit and open the doors for at least 24 hours to clear the drain. The longer, the better.
The second would be a failed defrost heater in the coils. This is not your typical defrost heater. It is embedded in the coils and it is a wire instead of an actual element. It requires a tech to replace it because of this design.
Try the defrost drain fix first and see if that clears it up. That is the most common problem on this unit anyways. When you plug the unit back in to let it run, open the door and hold the door switch in for 1 minute. Listen for the fan to engage just to confirm it is running.
Let me know if you have any other questions for me. I will be on for the rest of the night.