The air damper, baffle or diffuser is the device that balances the air flow from the evaporator fan housing. This normally is a mechanically controlled baffle or flapper that controls the amount of cold air entering the fresh food compartment. If the baffle is damaged or the linkage to the control knob is damaged, then you may not have enough cold air entering the fresh food compartment resulting in higher than normal temperatures. This baffle or damper will be located where the cold air enters the fresh food compartment.
The housing that the baffle or damper is encased in is often made of plastic with a Styrofoam lining and may have a foam seal to prevent air leakage. Care should be used when inspecting the housing to avoid damage to the seal or the Styrofoam. Locate the baffle/damper and operate the control knob to verify that the linkage is intact and that it moves freely. If the damper/baffle is stuck in the closed or nearly closed position or does not move freely, then it should be replaced. Some refrigerator models use a temperature sensing bulb attached to this control that will automatically adjust the control for changes in the interior temperature. Make sure that the sensing bulb is in the correct position and is not damaged.