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That frost build up prevents the air from circulating into the refrigerator from the freezer which is why the refrigerator isn't cooling. This means that you either have a defrost problem or the fan inside the freezer behind the back panel isn't running and needs to be replaced. Pull everything out of the freezer and remove the back panel. If the fan isn't running then replace it. If there is a lot of frost build up, then use a hair dryer to thaw it all out. You will then need to test continuity through the defrost heater and defrost thermostat. The defrost heater runs along the bottom of the silver coils and the defrost thermostat is the 1" round device clipped onto the top of the silver coils. If either part doesn't have continuity, you need to replace it or it stops the unit from thawing on it's own.
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