Performed preventative maintenance for a high efficiency Lennox furnace. I observed the system starting normally. I took a combustion analysis test and got the following readings, o2 is at 10%, co is at 3ppm and the system is operating at 93% efficiency. The amp draws for the blower motor and inducer motor are within normal range. I observed the system shut down normally once it reached the desired temperature. I removed and cleaned carbon off of the flame sensor. Cleaned out the burners and burner box. Checked all electrical components needed for heating. The homeowner recently replaced the air filter and water panel on the humidifier. The system is operating properly at this time.