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 14%, co is at 2ppm and the system is operating at 81% efficiency. Observed the system shutting down normally. 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 Merv16. Checked all electrical components needed for heating. The system is operating properly at this time.