Performed preventative maintenance for a Lennox furnace model #EL280UH090E36B-01, serial #5919K18008. I powered the system down and removed the access panels. Removed and cleaned carbon off of the flame sensor and cleaned out the burners and burner box. The homeowner recently replaced the air filter size 16x25x1. Checked all electrical connections to ensure that they are secure and in good condition. Checked the pressure switch for clogs. I powered the system back up and called for heat. Took a combustion analysis test and received the following readings, o2 is at 12.0% and the co levels are 13ppm. The inducer motor amp draw is at .13 and the blower motor amp draw is at 4.4. The system is operating properly at this time.