Completed 2 system furnace repair and preventative maintenance check. Replaced filter, checked electrical and safety switches all ok at this time. Upstair system has high oxygen content and suggest to do a combustion calibration to improve efficiency of furnace. Upstair has weak flame safety sensor. Found downstair system with worn ignition and weak flame safety sensor. Suggest apco UV light for bacteria and mold prevention. Both system heating at this time