Method and apparatus for circuit fault detection with boiler water level detection system

A fault detection circuit in a boiler-water level system includes a dual-frequency signal generator which develops two AC components with no associated DC component. The two frequencies are mixed and sent through an impedance matching circuit to match the impedance of the signal generating portion of the system with the impedance of the boiler water under measurement. The impedance-matched signal is then directed to two legs, one leg directed through one of a plurality of electrode probes and then to a first filter circuit, and the other leg is directed a second set of filters. In combination, the filters pass either the higher or the lower of the two frequencies to determine an open or short condition in the level sensing circuitry, as well as a steam vs. water condition.