I have an American Standard model TUD120C954A0 120,000 BTU gas furnace. When the thermostat is calling for heat the glow bar come on but gas does not flow the gas valve that is in the furnace. The rest of the appliances get gas. The red light on the controls is saying that its calling for, but it just cycles threw. Is my gas valve bad in the furnace or is it one of the safety switches.