The Jura E-J-S-WE Water Level Sensor MK04U-BV15352 is secured to the internal housing of the coffee machine. When the magnetic float of the Water Tank contacts the Water Level Sensor, the CPU is notified that the Water Tank is filled and secured properly. If you receive a "Fill Water Tank" message, check to see if the magnetic float in the Water Tank is stuck. If it floats freely, then this sensor may need to be replaced.