Performed preventative maintenance on a Lennox furnace. Upon arrival I watched system cycle on with a call for heat. I took a combustion analysis test and got the following results: O2 is 5.9% and the CO is 4ppm. While system ran, checked amps on the inducer motor and blower motor and both are in good range. I checked and cleaned flame sensor of any carbon build up. Checked all vinyl tubing to ensure no clogs or leaks in the system. Checked operation of condensate pump. Checked all electrical components to ensure the connections are good and tight. Checked the drain lines to ensure no clogs. Changed filter sized 16x25x5. Watched system cycle off when set temperature was reached. Returned thermostat to homeowners desired temperature. System is working properly at this time. *** Office to call customer for payment*** Model # CX35-36B-6F-1 Serial # 6016j17843