During a furnace tune-up service, a licensed technician will measure the gas pressure of your furnace, inspect the flame sensor, perform an audit of the combustion process, and clean the gas burners. To ensure it’s safe to operate your furnace, a licensed technician will perform an inspection of the Carbon Monoxide (CO) rating of your furnace. The Carbon Monoxide (CO) rating of a furnace will range from 0 to 100 ppm during the heating cycle.