The Sensor is built in the body of the machine. The oppositepart is a Neodym-magnet. This Magnet is permantly in the watertank. The water has permanty contact to the water we drink. This water destroys the neodym magnet an destroy the surface. This is dispicable black coorodes thing. You can changhe easely the magnet.
You need a long grill tong an pry the plastic sensor upwards our of the tank. It was holded with a metallclasp. Open the little plasticbox and change the magnet. rebuild it and drink the water.
A good decision is to change everytime the water in the tank. Leave the tank empty without water. Flush the container bevor making coffe with water.