If a gas furnace locks out while attempting to light and measures 19 VAC at the gas valve, what is the likely cause?

The situation described involves a gas furnace that fails to light properly and measures 19 VAC at the gas valve. The fact that the voltage reading is 19 VAC suggests that the electrical supply to the gas valve is not adequate for it to operate correctly. Gas valves typically require a minimum of around 24 VAC to open and allow gas to flow into the burner assembly.

In this context, if the electronic board were functioning correctly, it would supply the appropriate voltage to the gas valve. A fault in the electronic board can lead to insufficient voltage being sent to the gas valve, preventing it from opening and causing the furnace to lock out as it attempts to ignite. Therefore, the likely cause of the issue, given the insufficient voltage reading, is a fault in the electronic board that regulates power to the gas valve.

Undoubtedly, other options could lead to ignition problems, but the specific scenario of measuring 19 VAC is a clear indicator that the electronic board's failure is directly impacting the operation of the gas valve.