Microwave shows F4 and will not start or stops shortly after starting.

Safety First: Disconnect power before removing any covers or panels.

• Clean the cavity: Wipe down the interior, especially near the humidity sensor (often located on the upper side wall or ceiling). Remove grease and food splatter that may coat the sensor.
• Ventilate: Leave the door open for an hour to allow moisture to dissipate, then test again.
• Inspect sensor: With power disconnected, remove the outer cover and locate the humidity sensor. Check for obvious damage, corrosion, or disconnected wiring.
• Reconnect harness: Reseat the sensor connector on both the sensor and control board ends if accessible.
• Replace sensor if needed: If cleaning and reseating do not clear F4, replace the humidity sensor assembly. If the error persists with a new sensor, the main control board is likely defective and should be replaced by a technician.