OK perfect. So you are correct that you definitely have a defrost problem, but there is really no way to test the timer to make sure the timer is working because you can't watch it for 12 hours and make sure it's counting down on it's own and going into the defrost cycle. That being said, there are 3 possibilities that can cause the problem--the defrost heater, the defrost thermostat, and the defrost timer. You can actually test the thermostat and the heater if you have a multimeter. To do this, pull everything out of the freezer and unplug the refrigerator. Remove the lower back panel to access the defrost heater running under the bottom of the silver evaporator coils
(where all the frost is building up). Unplug the heater and test continuity. You should have between 5 and 30 ohms if the heater is good. The defrost thermostat is the quarter sized round device clipped onto the top of the silver evaporator coils. Unplug and test continuity through this part. You should have between 0.1 and 1 ohm resistance through the defrost thermostat if it's good. If either one of them doesn't have continuity, replace the part. If they both test good, you'd need to order and replace the defrost timer because that's the only part you can't test for certain.
Just as a general reference, the defrost thermostat is usually the part that fails which is part number 5303918202. Below is a link to the defrost thermostat since that is the likely part you'd need to replace.CLICK HERE
Now I did want to say that if you unplug the unit and let it thaw out or remove the back panel and use a hair dryer to thaw out all the frost quicker, it will work for a week or two, but the frost will build up again if you don't replace the defective part.
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