Dear,
I have an auto defrost Kenmore Refrigerator for 3 years, and since last month a layer of ice has been formed on the bottom of the freezer section (the top part of the Refrigerator). What is the most likely cause of the problem? The model number is: 501-64022.
Thank you very much for your help.

The most likely culprit is a faulty defrost timer switch. That part goes a lot and the life of a new timer switch is about 4 years.