Complete 3 system furnace repair and preventative maintenance check. Changed filters. Ran static pressure tests on all systems, pressures did not exceed system max ratings. Performed combustion analysis on both furnaces: the basement furnace CO levels are low and O2 levels are within range; I found the attic system to be slightly under fire (CO levels low and O2 levels are high). Tested all electrical and mechanical components, and all readings are nominal.