Check that the float is not hung up, this could be from a snail or crab or algae, or it could be the magnet holder is tilted, since the float is magnetic, a magnet near by or a misalignment of the magnet holder can also cause a problem. The outside magnet is smaller and should line up with the bottom of the bigger inside magnet the sensors mount on. The float should only be installed in the larger "c" on the magnet holder. Magnetic fields near the controller are another possible cause and I would suspect any new equipment added or moved between when it worked and when it did not. The other possibility is water damage to the controller, this usually is indicated by both the too high and too low lights being on.