DIAGNOSIS The humidity sensor is faulty or contaminated. It's a small sensor behind the inside front wall that detects steam. Grease or debris blocks it.
FIX Clean the sensor area. Unplug the microwave. Locate the small plastic vent/sensor cover on the inside front wall, near the top. Wipe it gently with a damp cloth. Do not use abrasive cleaners.
IF THAT DIDN'T WORK The sensor itself has failed. The sensor is part of the main control board. The entire board, part WB27X11334, must be replaced to fix the sensor cooking.
HEADS UP Sensor cook requires the correct container. Do not use metal or containers that don't allow steam to escape. The cavity must be cool when you start.