Microwave shows 'E-21' and may not start or may shut off quickly after starting.

Safety warning: High voltage and microwave radiation hazards exist inside the unit. Unplug the microwave and do not remove the outer cabinet unless you are trained. Many magnetron-related repairs should be done by a qualified technician.

• Allow cooling: If the microwave was used for a long period, let it cool for 30–60 minutes with the door open, then try again. Overheating can temporarily trip thermal devices.
• Check ventilation: Ensure that vents are not blocked and that there is adequate clearance around the microwave, especially for built-in and OTR models.
• Inspect filters: Clean or replace grease filters and charcoal filters (for OTR models) to improve airflow.
• Professional service: If E-21 persists, the magnetron thermostat or magnetron assembly may be faulty. These components are near high-voltage circuits and waveguides and should be tested and replaced by a professional.