Abstract: A water level gauge with fault detector comprises a column for containing a level of water with a plurality of light sources on one side of the column and a plurality of detector assemblies on an opposite side of the column. Filters are utilized to project red and green light through the column. Each detector assembly includes a red detector and a green detector with red and green filters respectively. Due to the difference in offset of light beams transversing the column at an oblique angle through water and through air, the green detector is activated by green light when the green and red light passes through the water in the column and the red detector is activated when the light passes through air in the column, that is, above the water level in the column. By a circuit connected to each detector assembly, it is possible to discover the level of water in the column.