Microwave display shows E04 and unit will not start; sometimes accompanied by a loud relay click and immediate stop.

Safety warning: Inverter-based microwaves use high-frequency high voltage. Internal components are extremely dangerous. Do not attempt board-level repair unless qualified.

Basic checks:

• 1. Power reset: Unplug the microwave for at least 5 minutes to allow internal capacitors to discharge partially and the control to reset. Plug back in and test.
• 2. Check outlet and supply: Verify the wall outlet is properly grounded and not shared with heavy loads. Avoid extension cords.
• 3. Listen for abnormal sounds: If you hear rapid clicking, buzzing, or arcing when starting, unplug immediately.

Technician-level checks:

• 4. Inspect inverter board: Remove the cabinet and visually inspect the inverter module for burnt components, cracked solder joints, or blown fuses.
• 5. Test magnetron load: Disconnect the magnetron from the inverter and test the magnetron for shorts. A shorted magnetron can trigger E04.
• 6. Replace inverter module: If the magnetron tests good but E04 persists, replace the inverter power board with the correct Westinghouse part.

Because of the complexity and danger, E04 faults should generally be handled by a professional technician.