The evaporator fan blows cold air into the freezer and from there it vents into the refrigerator. Occasionally the vents between the freezer and refrigerator can become clogged with ice, food or other debris. In most refrigerators the cold control for the refrigerator opens and closes these vents. That mechanism may become inoperative resulting in the vents becoming stuck open or closed.
Before checking for airflow restrictions, unplug the refrigerator or shut off the power at the fuse box or breaker panel to avoid an electrical shock hazard.
Inspect the vents to determine what is preventing the free flow of air. An overcrowded refrigerator or freezer may be the cause. In other cases the vents may need to be cleaned or ice melted away. To remove a build up of ice, use a hair dryer set to "low". Using a higher setting may damage the freezer. CAUTION: Do not let melting ice drip onto the hair dryer.
In some models, the vent is located under the temperature control console. The housing either snaps into place or is held in place with screws. Remove the screws, or gently depress the retaining clips with a small screwdriver. Allow the housing to hang by its wiring. A freezer vent control may also have to be removed to access the vent. In some freezer-on-top models, it may be necessary to remove the floor of the freezer to inspect for obstructions.