If your RS21 fridge evaporator defrost sensor has failed you will find an ice build up at the rear of the evaporator cover, similar to the image below
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This image shows the fridge evaporator severely iced up due to failure of the evaporator defrost sensor, part number DA32-00006W. You may consider replacing part number DA47-00095E - thermal fuse. The thermal fuse can fail as a result of the defrost sensor failure. If you have no means to test the circuits of these components, we would recommend you purchase these parts as a
service kit to include DA32-00006W and DA47-00095E