Performed preventative maintenance for a Lennox furnace. Observed the system starting normally. Took a combustion analysis test and got the following readings, o2 is at 8.8%, co is at 1 ppm and the system is operating at 87.2% efficiency. Blower amp draw is at 3.2. Removed and cleaned carbon off of the flame sensor. Cleaned out the burners and burner box. Checked all electrical components needed for heating. The system is operating properly at this time.