COSTARŽ 24VC-e carbon monoxide detecting ventilation fan controller is designed for use where ventilation fans are needed to prevent dangerous accumulations of carbon monoxide (CO) from engine exhaust or combustion appliances. The COSTARŽ 24VC-e lowers operating costs by activating the fan only when required to maintain good air quality while enhancing safety by alarming if dangerous CO conditions are detected. The COSTARŽ 24VC-e features an electrochemical sensor and a microcomputer-based CO detection circuit. The microcomputer provides overall supervisory control, measures CO level and calculates CO dose. It also self-tests the internal electronic circuit and signals a fault warning if a malfunction is detected. The CO sensing element is replaceable every 3 years. Each individual sensor is factory calibrated; no field adjustments are necessary or possible except for sensor replacement. A signal indicating sensor replacement is given when necessary.