BLowing warm air

Refrigerator & Freezer Repair
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1

refrigerator works fine when it is blowing cold air however, after a few hrs it starts blowing warm air and heats up to 60 degrees or so. We turn it off and unplug it to reset. Sometimes that works and sometimes it doesn’t. It seems to be getting stuck in the defrost mode. What do we need to replace in order to fix the problem. Thank you, Carol