Microwave shows E3 and stops heating; may beep repeatedly and cancel the cycle.

Safety first: Unplug the microwave. Internal components can store lethal voltages. Do not touch the high-voltage capacitor, transformer, or magnetron terminals unless you are trained and know how to discharge them safely.

• Check for blocked vents: Ensure all exterior vents are clear of dust, grease, and obstructions. Move the microwave away from walls to allow proper airflow.
• Let the unit cool: Wait at least 30 minutes, then plug back in and test. If E3 clears temporarily but returns, overheating is likely recurring.
• Inspect cooling fan operation: Start a short cook cycle with a cup of water. Listen for the internal cooling fan. If you do not hear the fan running when the magnetron should be on, the fan motor or its wiring may be faulty and must be replaced.
• Check thermal cutouts (advanced): Remove the outer cabinet. Locate the thermal cutouts on the magnetron and on the top of the cavity. With power disconnected, test each thermal cutout for continuity using a multimeter. Any that read open (no continuity) must be replaced with the exact Oster-specified part.
• If thermal cutouts and fan are good: The magnetron may be failing and drawing excessive current, or the control board may be misreading the thermal circuit. These repairs should be handled by a professional technician.