Question

How does the c4 cassette toilet level gauge work? Mine always shows empty.

Answer

There is a magnet on the float arm inside the tank, when the water level in the tank rises the magnet hits the sender in the cassette aperture and it reads full. If this is not working, place a magnet next to the sender and see if it shows a reading, if this makes it work again then the magnet has dropped off inside the tank, if it does not then the sender is faulty. Without knowing which model toilet you have I cannot tell you where the sender is as they are in different places.