Microwave displays F4 and will not start or stops mid-cycle.

Safety first: Disconnect power before opening any panels.

Clean and inspect:

• The humidity sensor is typically located in the upper area of the cavity or behind a small grill/vent inside the oven.
• Clean the sensor opening and surrounding area with a mild detergent and soft cloth. Built-up grease or residue can affect readings.
• Do not use abrasive cleaners or soak the sensor.

Access wiring (for over-the-range models):

• Remove the top grille and control panel to access the sensor wiring harness.
• Locate the small sensor module and its connector. Check for loose plugs, corrosion, or damaged wires.
• Reseat the connector firmly.

Test operation:

• Restore power and attempt a sensor cook or reheat function. If F4 reappears immediately, the sensor is likely defective.

Corrective action:

• Replace the humidity / steam sensor if cleaning and reseating connections do not resolve the error.
• If a new sensor does not fix the issue, the electronic control board may need replacement.

Note: You can often still use manual time cook modes if the error only appears during sensor functions, but the underlying fault should be corrected to avoid further issues.