Complete 3 system furnace repair and preventative maintenance check. Find upstairs 2-year-old Lennox heating system operating normal with in manufacturer spec. No concern at this time. Change filter customer provided. Find main system carrier 5-year-old system operating high co level and overheating furnace exchanger. Complete combustion calibration to help reduce heat and reduce co level to 20ppm and 82% efficiency. Did not change filter. Find the basement system 21 years old operating with badly worn ignition and overheated control board resistor and can damage control system. Customer does not want to repair the basement system at this time. did not change filter.