The interior light in most freezers, and the fan in some, is controlled by a door switch. When the door is closed, the switch is depressed and the interior light goes off (yes, it really does) and the fan resumes normal operation. If the door is misaligned or the switch malfunctions, the freezer may become warm as a result of the non-operation of the evaporator fan and the heat generated by the interior light.
Before testing the switch, unplug the freezer to avoid an electrical shock hazard.
Locate the switch and remove it by removing the retaining screw (if any) or by prying it with a putty knife. Place a piece of masking tape over the blade of the putty knife to prevent damage to the refrigerator. Gently pull the switch from its housing and expose an inch or two of wire.
The switch will be connected by two wires (four wires if it controls additional features). Label the wire placement on the switch before disconnecting the wires. The wires are connected to the switch with slip on connectors. Firmly pull the connector off of the switch terminal (do not pull on the wire). You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be replaced.