Abstract: A carbon monoxide detector for detecting the level of carbon monoxide in a furnace supply duct. The carbon monoxide detector includes a carbon monoxide detector that has a housing. The housing is coupled to an exterior surface of a furnace supply duct of a furnace. The carbon monoxide detector also has a probe assembly that extends outward from the housing into the furnace supply duct for detecting a level of carbon monoxide in the furnace supply duct. The carbon monoxide detector further has a control unit. The control unit is operationally coupled with a limiting switch of the furnace. The control unit provides a signal to the limiting switch upon detection of a carbon monoxide level above a pre-determined level whereby the limiting switch is activated to shut off the furnace.