Performed preventative maintenance on a standard gas furnace. Inspected all the electrical components. Customer replaced the standard air filter 14x25x1. Confirmed proper high and low voltages. Cleaned the burners, flame sensor and burner compartment. Tested the amp draw for the blower and inducer. Both are operating within the manufacturer specs. Performed a combustion analysis on the furnace. System is operating at 80.5% efficiency with a CO level of 23ppm. System is operating properly at this time.