Bryant gas furnace fails to ignite. LED displays code 34: Ignition proving failure. Hot-surface ignitor glows, but I never hear any gas. The main gas line is open. Gas hot water heater and another gas furnace work fine, so gas is likely OK. I assume that because the flame never ignites that the flame sensor can't be the problem. What could cause the gas to not flow to the ignitor? Furnace is 12 years old. No other prior service problems.