Find the part that failed
Every guide here starts from something you can see, hear or time from outside the furnace — and ends at the part that caused it.
When it will not lightWhy Won't My Furnace Ignite?Follow the furnace startup sequence — inducer, pressure switch, ignitor, gas valve, flame sensor — to find where it stops and which part is at fault.
When you want to know what normal isYour Furnace Sequence of OperationWhat a gas furnace does from the thermostat call to the blower, step by step — the voltages, the timings, and the safety device gating each stage.
When it lights, then quitsFurnace Starts Then Shuts OffA furnace that lights then quits tells you something by how long it ran: seconds, minutes, or a hard lockout each point at a different failure.
Before any of it goes wrongBlocked Furnace FiltersA neglected filter starves the system of air, overheats the heat exchanger, and takes out parts your warranty will not cover. Inspect it monthly.
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