A gas furnace will not light and locks out during the process with 19 VAC at the gas valve. What should the technician conclude?

In the situation where a gas furnace is not lighting and locks out with 19 VAC at the gas valve, the correct conclusion is that the secondary of the transformer could be faulty.

In a gas furnace, the transformer is responsible for stepping down the voltage to the appropriate level required for various components, including the gas valve. If the furnace is showing 19 VAC at the gas valve, this indicates that there is some voltage present; however, it may not be sufficient to fully open the gas valve or allow it to operate as intended. A faulty transformer could be delivering inadequate voltage or current, which would lead to the gas valve not functioning properly even though it registers some voltage.

Thus, if the transformer is not providing the necessary output, it can cause the system to lock out due to the gas valve not opening, preventing ignition. This emphasizes the importance of confirming the health and operational capacity of the transformer in troubleshooting the furnace's failure to ignite.