Complete 2 system furnace repair and preventative maintenance check. Left filter for the owner, checked electrical and safety switches all ok at this time. Found both furnaces with worn ignition but working at this time. Found downstair with high carbon monoxide and low oxygen. Downstair furnace is getting to much fuel and causing improper combustion which creates high carbon monoxide. Did a combustion calibration for downstair system from 6.9% oxygen to 8.7 with carbon monoxide reading of 22ppm. Overall both system heating normal