MACURCO CM-21A is a low voltage, dual relay Carbon Monoxide (CO) gas detector and controller. The CM-21A uses an electronic detection system to measure the concentration of CO and provide automatic exhaust fan control to help ensure a safe environment in parking garages or maintenance facilities. The CM-21A is factory calibrated and 100 % tested for proper operation, but can also be calibrated in the field.********DISCONTINUED********** Replaced by CM-6