The defrost heater commonly fails in many models of refrigerators. They are very easy to check and replace. The hardest part is usually getting to it. The defrost heater is going to be located on the evaporator coil. It's function is to melt the ice that builds up on the evaporator coil. If the ice is allowed to build up first the cooling in the fresh food section starts to fail then the freezer section will also. If the defrost heater has continuity check to make sure it is getting power. It should have about 120 volts when put into defrost mode. Most refrigerators can be put into a forced defrost but the procedure varies from model to model (check service manual). If it has no power or has continuity the problem is elsewhere.
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