F4 appears and the freezer repeatedly enters or exits defrost mode at the wrong times, causing temperature swings.

Safety first: Unplug the freezer before working on the control board or defrost wiring.

• 1. Reset control: Disconnect power for 15 minutes to reset the control logic. If F4 returns, continue.
• 2. Verify defrost sensor: Test the defrost NTC sensor on the evaporator for correct resistance and operation. Replace if faulty.
• 3. Inspect defrost relay: On the main PCB, locate the relay that switches the defrost heater. Look for signs of overheating or sticking. A relay stuck closed can cause excessive defrosting.
• 4. Check wiring: Inspect the harness from the PCB to the defrost heater for insulation damage or shorts to chassis.
• 5. Replace PCB if needed: If sensors and wiring are good, the defrost control logic or relay on the PCB is likely defective. Replace the main control board.
• 6. Monitor temperatures: After repair, monitor the freezer over several days to ensure defrost cycles are normal and F4 does not reappear.

Improper defrost control can lead to heavy ice buildup or warm temperatures; address F4 promptly to protect stored food.