Abstract: A dual-beam photoelectric switch and method adapted for detecting overheight vehicles that emits and detects red light along an optical axis, and emits and detects infrared light along a parallel and oppositely oriented optical axis. A fault detector receives signals from the light detectors, processes the signals in accordance with a predetermined logic and produces a pair of outputs indicating the fault status of the light detectors. An alarm mode component receives the signals of light detectors and the fault status outputs, enables and disables recognition of signals of the light detectors in relation to the fault status outputs, and triggers certain outputs in accordance with a predetermined logic.