Which switch must operate properly before the ignition sequence of a gas furnace starts?

The correct answer involves understanding the sequence of operations in a gas furnace. For a gas furnace to safely commence its ignition sequence, several safety interlocks must be verified. The differential air pressure switch is one of these critical components. It ensures that there is adequate airflow to the furnace, indicating that the combustion process can take place safely. If the pressure switch does not signal that sufficient airflow is present, the ignition sequence will not initiate, preventing possible malfunctions or dangerous situations like overheating or backdrafting.

This safety feature is essential because it helps in determining whether the exhaust system is working correctly and that fresh air is entering the combustion area as needed. If there is an issue with the airflow, the system will not proceed to ignite, protecting both the equipment and users from potential hazards.

Each of the other options—while vital to the overall function and safety of the furnace—does not directly govern the airflow needed for ignition as the differential air pressure switch does. Thus, it plays a pivotal role in the safe start of the ignition cycle by confirming that all parameters for safe operation are satisfied.