Performed preventative maintenance for a Lennox furnace model #SL280UH110V60C-05, serial #5917M08846. Upon arrival I called for heat and observed the system starting normally. I took a combustion analysis and received the following readings, o2 is 13.1% and the co levels are 11ppm. The inducer motor amp draw is .23 and the blower motor amp draw is 4.35. I powered the system down and removed the access panels. I 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. Replaced the air filter size 16x25x1 and checked all electrical connections to ensure that they are secure and in good condition. The system is operating properly at this time. Please call the homeowner in a week to go over estimate options.