Completed 2 system furnace repair and preventative maintenance check. Replaced filters, checked electrical and safety switches all ok at this time. Flue pipe is foil wall for both furnaces and is good. Found worn ignition and weak flame safety sensor for the downstairs system. Replaced both parts. Did a combustion test and found high oxygen content for both furnaces. Did a combustion calibration for both furnaces for safety and efficiency. Installed low carbon monoxide detectors for upstair and downstair. Tested and everything heating normally at this time.