Performed yearly maintenance and safety checks on 2024 american standard furnace. Made call for heating. 21 ppm co in flu. 0 ppm co in supply. 30 degree deltat. Furnace cycled into second stage. Running with 18 ppm co in flu. 0 ppm co in supply. 35 degree deltat. Cleaned flame sensor. Blew out trap. No issues at this time.