Freezer shows Fb and interior light or door switch functions abnormally

Safety first: Disconnect power before working around door wiring or switches.

• Identify door sensing method: Many Hitachi freezers use a magnetic reed sensor in the cabinet and a magnet in the door; others use a mechanical plunger switch.
• Check magnet alignment: Ensure the door magnet lines up with the cabinet sensor area. Adjust door or replace magnet if missing.
• Test mechanical switch: For plunger switches, press the switch by hand and listen for a click. Test continuity with a multimeter; replace if it does not change state.
• Inspect wiring: Check the harness from the door switch/sensor to the main control PCB for damage or loose connectors.
• Replace sensor/switch: Install a new Hitachi door switch or magnetic sensor if faulty.
• Control PCB: If the sensor tests good but Fb remains, the main control PCB input circuit may be defective.