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If the thermostat were bad, nothing would run including the fan motor or the compressor or anything else. The thermostat is what tells the fan and everything else to turn on and cool. That means that since the compressor is running and the freezer is trying to cool, either the fan motor itself has failed, or the defrost thermostat is cutting power to the fan motor. Clipped onto the top of the silver evaporator coils is a 1" round device with 2 wires going to it. That is your defrost thermostat. Check continuity through this part and if it has not continuity through it, it will cut power to the fan motor so it needs to be replaced. Keep in mind that this is a thermostat, so it will only close if the evaporator coils are frosted and below 32 degrees. If the unit is room temperature and the silver coils have no frost, it could take 30 minutes for the thermostat to cool down enough to close or so.
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