19 yr old aire-flo furnace is over heating and cutting out on high temperature limit. Changed dirty filter and unit is still overheating with an 82 degree temperature rise. Gas pressure 2.9 inwg, combustion analysis had elevated carbon monoxide levels in flue gas(118ppm). Return duct is very dirty. Blower wheel, secondary heat exchanger, evaporator coil, and duct work would need cleaned to get furnace running within an acceptable heating range. Furnace also seems to have an issue at start up with pressure switch or control board. Draft motor kicked on for a few seconds and shut back off a couple of times before eventually stay running to close the switch. Given that furnace is past average life expectancy and high cost of repair I would recommend getting a new system installed. Estimate for quote on new equipment is scheduled Thursday feb 23rd 9-11am.