Safety: Unplug the microwave before servicing. Do not touch the high-voltage section.

Locate humidity sensor: On many Viking over-the-range microwaves, the humidity sensor is located behind a small grille or opening in the top or side of the cavity interior.
Clean sensor area: Wipe the interior around the sensor opening with a mild degreaser and a soft cloth to remove steam and grease buildup that can affect readings.
Access sensor: Remove the outer cover or control panel as needed to reach the sensor module. Inspect the sensor body and wiring harness for damage or corrosion.
Check connections: Ensure the humidity sensor connector is fully seated on the sensor and on the Viking control board.
Replace sensor if suspect: If F4 persists after cleaning and reseating, replace the humidity sensor with the correct Viking part. Route the harness as originally installed.
Control board check: If a new sensor does not clear F4, the main control board may have a failed sensor circuit and require replacement.

Microwave displays F4 and will not start a cook cycle; may appear immediately when power is applied.

Possible Causes