Performed preventative maintenance for maintenance for a York gas furnace. Air filter 20x25x5 was just replaced no need to do it today. Cleaned the burners, burner box and flame sensor. Inspected all electrical components to ensure they are intact and operating properly. Checked the gas piping. Performed a combustion analysis on the furnace. The system is operating at 95% efficiency with the CO level at 14ppm. The system is operating properly at this time.