Microwave displays F4 and will not run; sometimes appears after a loud pop or during cooking.

Safety first: Disconnect power before removing any covers. Do not operate the microwave with covers removed.

• Locate humidity sensor: On many Fisher & Paykel microwaves, the humidity sensor is located behind the control panel or near the top of the cavity, often in a small housing with an opening into the cavity.
• Clean sensor area: If accessible, gently clean around the sensor opening with a dry cloth to remove grease and residue. Do not spray cleaner directly into the sensor.
• Inspect wiring and connector: Check the sensor harness for damage and ensure the connector is fully seated on the control board. Clean any light corrosion with isopropyl alcohol.
• Test sensor (if specs available): Using the service manual, measure the sensor's resistance or voltage output. Replace the humidity sensor if readings are out of specification.
• Replace sensor or control board: If cleaning and reseating do not clear F4, replace the humidity sensor assembly. If the fault persists with a new sensor, the main control PCB is likely defective and should be replaced.