Abstract: A system for sensing smoke and fire, primarily through the sensing of associated carbon monoixide and optionally with supplementary heat sensing, and for sensing hazardous gases and vapors. The system senses the presence of carbon monoxide, even in the absence of fire and is capable of sensing the presence of hydrogen and hydrocarbon vapors such as methane and propane. Additionally, the volatile vapors of paints, varnishes and other household and industrial substances are detectable. The system also includes circuitry and equipment for notification and warning of such detection, together with circuitry and equipment for providing emergency operation in the event of normal power failure. Further circuitry and equipment for self-monitoring of system remote wiring and sensor integrity, and associated notification means for such integrity checks is included.