To test the gas valve pull one of the wire of the valve while the heater is running. If the heater stops the control board was calling for heat. If with the wire off the gas valve may be mechanically stuck open. Not likely because the gas valve has 2 spring vales in it and the likely hood that both could be stuck open is slim.
Another thing to check. There are 2 klickons on the face of the heater that are wired in series. Pull any wire in that series and see if the heater stops. If it does stop the high limit switch has welded closed. They are replaced in a set.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles