Performed preventative maintenance for a Lennox furnace model #Sl280uh090v36b-03, serial #5914H32223. Upon arrival the system was running. I powered the system down or the furnace and removed the access panels. Removed and cleaned carbon off of the flame sensor and cleaned out the burners and burner box. Checked the pressure switch for clogs and tested the drain line and condensate pump with several bottles of water to ensure that there are no leaks or clogs. Checked all electrical connections to ensure that they are secure and in good condition. The filter was replaced by the homeowner a couple of weeks ago. Merv11 size16x25x5. I powered the system back up and called for heat. Took a combustion analysis test and received the following readings, o2 is 17.2% and the other levels are 9ppm and the supply temp is 120°F. The inducer motor amp draw is within normal range. The system is operating properly at this time.