Abstract: A new Combination Carbon Monoxide Sensor and Furnace Shut-Off System for automatically terminating natural gas flow to a malfunctioning boiler or furnace which is producing dangerous levels of carbon monoxide. The inventive device includes a carbon monoxide sensor system for detecting carbon monoxide, and a control valve electrically connected to the carbon monoxide sensor system and connected within a gas pipe for controlling the flow of natural gas within the gas pipe. When the carbon monoxide sensor system detects a dangerous level of carbon monoxide surrounding the boiler or furnace, a solenoid is electrically activated for closing the control valve for stopping the flow of natural gas into the malfunctioning boiler or furnace.