The Testo 317-3 (0632 3173) Ambient CO Meter detects the presence of carbon monoxide (CO) in ambient air and warns the user if potentially harmful CO concentrations are reached. An exclusive self-test mode provides worry-free operation, as it will always verify that the CO sensor is functioning, anytime, anywhere, without the need for test gas. In addition, the self-check will ensure accurate CO measurements, without the need to “zero” out the unit in a fresh air environment. With programmable settings for both visual and audible alarms, as well as hands free operation, ambient CO testing has never been easier for furnace and boiler service and maintenance, weatherization audits or home inspections.