Microwave display shows E04 and the unit will not start; sometimes accompanied by a loud hum or clicking before the error.

Safety first: Unplug the microwave. Inverter-based microwaves use high-frequency high voltage and are extremely hazardous to service without training.

User-level checks:

• Power-cycle the microwave by unplugging it for 5–10 minutes, then reconnecting. If E04 returns immediately when trying to cook, an internal fault is likely.
• Ensure the unit is not overloaded with extension cords or low-voltage supply; plug directly into a properly grounded wall outlet.

Technician-level checks:

• Inspect the inverter board for burnt components, cracked solder joints, or blown fuses.
• Check wiring harnesses between the control PCB, inverter, and magnetron for heat damage or loose connectors.
• Test the magnetron for shorts that could overload the inverter.
• Replace the inverter module if it is confirmed faulty; in many Hoover models, the inverter and high-voltage section are integrated.

Note: Inverter repairs should be carried out only by qualified personnel. If E04 persists, arrange professional service.