Microwave runs but stops with E04 error; sometimes accompanied by a humming noise.

Safety warning: Do not operate the microwave repeatedly with this error; it can damage the transformer and pose a fire risk. Unplug the appliance immediately.

• Step 1 – External checks: Remove any metal objects or foil from inside the cavity. Inspect the waveguide cover (mica plate) for burns or carbon tracking; replace if damaged.
• Step 2 – Visual inspection: With power disconnected and outer cover removed, look for burnt insulation or arcing marks around the high-voltage transformer, capacitor, and diode area. Do not touch these components.
• Step 3 – Professional diagnosis: A technician should test the high-voltage diode and capacitor with appropriate meters and safety procedures. A shorted diode or capacitor must be replaced as a set with any damaged wiring.
• Step 4 – Magnetron check: If the magnetron is shorted to ground, it must be replaced with the correct AEG magnetron assembly. The technician should also verify that the cooling fan is working correctly to avoid overheating.