Microwave display shows E4 and unit will not start any cooking program.

Safety Warning: Always unplug the microwave before removing covers or control panels. Avoid touching high-voltage components.

Cleaning and reset:

• 1. Power cycle: Unplug the RCA microwave for 5 minutes, then plug it back in to clear temporary sensor errors.
• 2. Clean interior: Wipe the oven cavity thoroughly with a mild detergent solution to remove grease and food splatter, especially near the top and sides where the humidity sensor is typically located.
• 3. Dry the cavity: Leave the door open for 20–30 minutes to allow moisture to evaporate, then test again.

Sensor inspection (intermediate DIY):

• 4. Access the control area: Unplug the unit. Remove the top grille (OTR) or outer cover (countertop) and then the control panel screws to gently pull the control panel forward.
• 5. Locate the humidity sensor: It is usually a small component mounted near the top of the cavity with a small harness going to the control board. Check that the connector is fully seated and free of corrosion.
• 6. Reseat connector: Disconnect and reconnect the sensor plug several times to clean the contacts. Reassemble and test.

If E4 persists after cleaning and reseating, the humidity sensor or main control board likely needs replacement. This is best handled by a technician to ensure correct part selection and safe reassembly.