The system consists of a magnet float and a sensor that activates when the float is at the bottom.
I have found two reasons the coolant level sensor fails.
The sensor gets dirty from scale and other grime and then sticks to the bottom. Sometimes cleaning the de-gas tank will free it up. However, the tank is difficult to clean because of all of the internal baffles.
What has given me limited sucess is to remove the de-gas bottle and soak in a vineager solution. Shaking and letting it soak. Repeat. Repeat.
The other cause is a water logged float. IE, coolant gets inside the float and it becomes too heavy to float. Note, the float is designed so that it will NOT float as well if the coolant concertration is not correct.
Again, what can happen is the float will stick to the bottom. Once freed up, it will float on its own. If the float is too water logged, it will never float and it can not be replaced (the de-gas bottle is built around it).