Performed yearly.maintenance and safety checks on rheem furnace. 13.2%o2. 34 ppm co in supply 42 degree deltat. Furnace cycled into second stage properly. Running with 8.65%o2. 52 ppm co in flu. 55 degree dletat. Condensate pump in operating order. Found dirty return. Presented video to customer and left fresh air Corp card. Left 16251 filter. Performed yearly.maintenance and safety checks on rheem furnace. 13.2%o2. 34 ppm co in supply 42 degree deltat. Furnace cycled into second stage properly. Running with 8.65%o2. 52 ppm co in flu. 55 degree dletat. Condensate pump in operating order. Found dirty return. Presented video to customer and left fresh air Corp card. Left 16251 filter.