The Water Level Sensor is located behind the water tank inside the machine. In conjunction with the float in the Water Tank, the Level Sensor detects when the water tank is filled, or needs water added. If you receive the message "Fill Water Tank" when the tank is filled, then check the Level Float. Sometimes mineral build-up causes the float to stick in the down position, signaling to the machine that the tank needs to be filled. Loosen the float by adding hot water and soap.