Diagnosis
The evaporator fan in the freezer compartment is likely obstructed by frost or has failed. This prevents cold air from circulating into the fresh food section, causing the freezer to work harder and not maintain temperature.
How to Fix It
Unplug the refrigerator. Remove all items from the freezer. Remove the rear interior freezer panel to access the evaporator coil and fan. Check for a solid block of frost on the coil. If present, thaw it completely with a hairdryer on cool setting. Manually spin the fan blade to check for obstruction or a seized motor. Clear any ice or debris. Reassemble and plug in. The freezer should reach 0°F (-18°C) within 24 hours.
If That Didn't Work
The issue may be a failed defrost system. The defrost heater, defrost thermostat, or main control board may not be initiating the defrost cycle. This requires a multimeter to test component continuity. The defrost heater should read between 20-50 ohms.
Heads Up
Do not use sharp tools to chip ice from the evaporator coils. You can puncture the refrigerant line. Allow it to thaw naturally or use only forced air.