Performed preventative maintenance for a high efficiency Lennox furnace. Observed the system starting normally. Took a combustion analysis test and got the following readings, o2 is 9.9%, co is 6ppm and the system is operating at 95.6% efficiency. Observed the system shut down normally. Removed and cleaned carbon off of the flame sensor. Cleaned out the burners and burner box. Replaced the air filter. Checked all electrical components needed for heating. The system is operating properly at this time.