I have been battling this for about a year now, the lower vent (that circulates warm fridge air into the freezer) freezes up, causing lack of circulation and the fridge warming up. This happens every month or two, I need to manually defrost. I replaced the thermistor (in the freezer), the thermostat (in the freezer), the main control board (on top of the unit), confirmed the fans in the freezer are spinning and the heating assembly has continuity. I also checked the door seals, they look fine. I tried plugging fridge side pressure equalizer hole and the opening for the water hose, dusted condenser coils. It appears there is too much moisture entering the fridge. The ice covers the vent hole on the freezer side, but there is no ice on the evaporator.