I've read numerous threads on this issue on the forum. Mine does not light up. If I put a magnet to the side of the cassette it attracts the magnet on the float switch and seems to move it up and down so I assume the float is working and it sounds like it is as well. No doubt someone will tell me if this assumption is incorrect. You can see the contact in the cassette compartment in the photo. The batteries are fine. It seems to me that the contact lines up almost level with the bottom of the spout and I doubt that it's ever been that full. I empty it daily. Does it really have to be that full to work? If so I'll just keep pouring water in and see what happens. I know the correct answer is to forget it and just empty it regularly which I will but the switch is there and it's frustrating. Lastly does it only go red when full or should there be a light on all the time?