Performed preventative maintenance for a Lennox furnace model #SL280UH090V36B-04, serial #5919408560. Upon arrival I called for heat and observed the system starting normally. I took a combustion analysis test and received the following readings, o2 is 10.5% and the co levels are 15ppm. The inducer amp draw is .18 and the blower motor amp draw is 4.9. I powered the system down and removed and cleaned carbon off of the flame sensor. Cleaned out the burners and burner box. Replaced the air filter size 16x25x1 and tested the drain line with several bottles of water. Checked the pressure switch for clogs and checked all electrical connections to ensure that they are secure and in good condition. The system is operating properly at this time.