Microwave beeps and shows E04; cooking will not start.

Safety: Unplug the microwave before cleaning inside. Avoid spraying cleaner directly into vents or onto sensors.

User-level steps:

• 1. Locate the humidity sensor: On many Breville microwaves, the humidity (steam) sensor is a small opening or grille on the upper interior wall or ceiling.
• 2. Clean the cavity: Wipe the interior walls and ceiling with a soft cloth and mild dish soap solution. Pay attention to the area around the sensor opening. Remove grease, food splatter, and condensation residue.
• 3. Steam clean: Place a bowl of water with a little lemon juice inside and run on HIGH for 3–5 minutes, then let sit closed for a few minutes. Carefully remove the bowl and wipe down the interior again.
• 4. Power reset: Unplug for 5–10 minutes, then plug back in and test an auto-cook or sensor-cook program.

Technician-level checks:

• Test the humidity sensor with a multimeter according to Breville specifications.
• Inspect the sensor wiring harness and connector at the control board.
• Replace the humidity sensor if readings are out of range.

If E04 persists after thorough cleaning and reset, the sensor or control board likely needs replacement by a professional.