No heat on C18M. Found control board not sending signal to gas valve to start igniter, replaced board. After replacing the board the gas valve would not open. It appears the board went bad and took the gas valve with it. Replaced gas valve and unit started up great. While this far into it I felt the igniter was due to be changed, looked terrible, so I replaced it to save a future service call. Tested furnace, running good, room raised 3 degrees while I was here.