Gas Furnace Sequence of Operation: Step-by-Step Exam Logic

The short field answer:
The exam wants you to think in sequence, not in parts. Typical gas furnace logic goes like this: thermostat calls for heat, inducer starts, pressure switch proves, ignitor warms up, gas valve opens, flame is proven, indoor blower starts after delay, and second stage may come on later if the demand continues. If proof fails at any point, the board retries or locks out.

Lennox service literature gives very explicit fault paths: pressure-switch faults point you toward inducer operation and vent restriction; low flame current points you toward flame-sensor microamps and grounding; limit faults point you toward firing rate, blockage, and airflow. Lennox also documents ignition timing details such as ignitor warm-up, gas valve opening, blower on-delay, and retry/Watchguard lockout behavior when flame is not proved.

Best exam trick:
When given a failure, ask: “What was the last confirmed good step?” That single question usually narrows the answer faster than memorizing ten parts in isolation.