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-Could you explain your situation a little more?
What is the model number of the refrigerator?
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With the information you have provided the most common reason for the failure is a problem with the self defrost feature of the refrigerator. This will cause ice to form on the evaporator coils that are located in the back of the freezer compartment. Defrost failure is often caused by a failed defrost timer. Click here for information on testing the defrost timer. If the defrost timer is not the problem the defrost heater or the defrost heater thermostat can also be the problem. These are located in the freezer compartment and can be accessed by removing the back panel inside the freezer compartment. These can be checked for opens with a volt/ohm meter.
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The following information was porovded by another expert
Most Sub-Zero's do not utilize a defrost timer on the ref side. Since the ref should not stay below 34Â° f, whatever frost accumulates during the "On" cycle of the compressor will melt during the "Off" cycle. (This is referrred to as "Off-cycle defrost"). The freezer side (if so equipped) will utilize either a mechanical timer or have a heated defrost controlled by the main bd and a defrost limit switch.
Thank you AccApp for the information.