Performed clean and check on gas furnace and humidifier. Conducted combustion analysis, measured temperature rise, inspected gas supply, burner, inducer and blower assemblies, tested all electrical components, cleaned condensate trap and inspected drains, cleaned flame sensor, changed filter and humidifier pad. Cycled system in heating. System is operating normally at this time. Customer stated that the outdoor unit is unlevel. Found pad sunken on one side. Notified customer that we will relevel AC during spring maintenance check.