DIAGNOSIS The humidity sensor (steam sensor) behind the vent grille is dirty or faulty. It detects steam to determine cook time.
FIX Unplug the unit. Remove the vent grille on the right side. Locate the small sensor, usually a plastic nub with holes. Clean it gently with a cotton swab and isopropyl alcohol to remove grease and dust.
IF THAT DIDN'T WORK The sensor itself may be defective. It's part of the main control board assembly. A full board replacement (WR55X10942) is the fix.
HEADS UP For sensor functions, always use microwave-safe containers that are wider than they are tall. A covered plate traps steam better than a tall, narrow bowl, giving the sensor a more accurate reading.