Customer signed up for a commercial single system maintenance. Gas valve at unit was left off by the gas company. Unit fired up after valve was turned back on. Filter is 1-16x25x1 that goes inside the furnace. I checked and cleaned the flame sensor and igniter. I checked the amp draw on the blower and inducer motors. I checked the blower capacitor and all electrical components. I checked the safties and confirmed proper sequence of operation. System is currently heating at manufacturers specifications.