Performed preventative maintenance for gas furnace per repair/maintenance agreement. Cleaned the burners, burner box and flame sensor. Inspected the gas piping and flue piping. Inspected all electrical components. Confirmed the proper high and low voltages. Replaced the standard air filter. The proper size filter for the slot is 16x25x1 if you have difficulty getting it in and out you can use a 20x25x1. You can upsize the filter to make it easier to replace versus trying to get your fingers in the get it out. it just makes it easier for installation and removal and will not affect the system operation. Filter should be changed every 30-90 days no more than 90 days because it can damage the system like it possibly did when the blower motor burned out. Performed combustion analysis on the furnace. System is operating at 83.9% efficiency with a CO level of 12ppm. System had a temperature rise of 54°. System is operating properly at this time. No charge on site due to prepaid repair agreem