I have a goodman gmp-125-5. The igniter ignites but the gas valve doesn't turn on. How do I trouble shoot to determine the problem ? I get a slow blinking light that I interpret to be a "1" on the mother board indicating a problem with ignition.

Follow the purple wire on gas control to the series of roll-out switches mounted on the burner enclosure. Press the center button on each - a "click" indicates the switch had opened. If operation is restored, observe operating sequence from start-up. If flame disturbance is detected when circulating blower starts/runs, have heat exchanger inspected.