DIAGNOSIS The humidity sensor isn't getting an accurate reading, usually because the interior isn't completely clean or you're using the wrong type of cookware. Steam from food is how it determines doneness.
FIX First, give the interior a deep clean, focusing on the ceiling and the back wall. The sensor is a small, round vent/hole on the back wall. Wipe it gently with a damp cloth—no abrasive cleaners. Always use microwave-safe containers that are wider than they are tall to allow maximum steam release. Never use sealed containers or ones with narrow openings. Cover food with a vented lid or paper towel, not plastic wrap.
IF THAT DIDN'T WORK If it's still undercooking, you might need to recalibrate the sensor. Run 1 cup of water in a standard measuring cup on the sensor reheat function. If it doesn't heat it properly, the sensor itself may be faulty, which would require a control board service.
HEADS UP The sensor works best with moist foods (vegetables, leftovers). It's less reliable for dry items like popcorn or reheating bread, which can burn before enough steam is generated.