Performed yearly maintenance and safety checks on 3rd story 2013 tempstar furnace. Made call for heat. 54 ohm ignition system. Cleaned flame sensor and flu trap. Left new 16251 filter. Furnace ran with12.8% o2. 31 ppm co in flu. 40 degree deltat. Furnace cycled into second stage 9%o2. 21 ppm co in flu. 0 ppm co in supply. No issues found at this time. Also performed yearly maintenance and safety checks on main basement 22 Bryant system. Made call for heat found 54 degree deltat. 12.5%o2. 13 ppm co in flu. 0 ppm co in supply. Furnace cycled into second stage properly running with 9.75%o2. 0 ppm co in flu. 0 ppm co in supply. 20254 filter in need of replacement. Offered replacement with filter on van. Customer stated he will replace himself.