Performed yearly maintenance on 23 payne furnace. Cleaned flame sensor. 49 ohm igntion. 10.6%o2. 46 ppm co in flu 0 ppm co in supply replaced 14251 filter. 45 degree deltat. Performed yearly maintenance on older Garage furnaceunit is running safely at this point in time. No issues at this time. 46 ppm co in flu. 0 ppm co in supply. 59 degree deltat. Unit is in rough overall shape. Will most likely need to be replaced in coming years.