Performed preventative maintenance for a Coleman furnace. I observed the system starting normally. I took a combustion analysis test and got the following readings, o2 is at 17%, co is at 3ppm and the system is operating at 77% efficiency. The amp draw for the inducer motor and blower motor are within normal range. Observed the system shut down normally once it reached the desired temperature. Removed and cleaned carbon off of the flame sensor. Cleaned out the burners and burner box. Checked all electrical components needed for heating. Replaced the air filter 16x25x1. The system is operating properly at this time. Performed preventative maintenance for a carrier furnace. I observed the system starting normally and took a combustion analysis test and got the following readings, o2 is 15%, co is at 9ppm and the system is operating at 82% efficiency.