From the Blog · College Grove, TN
Why Won't My Furnace Ignite?
When a furnace clicks, starts its inducer fan, then shuts down without heat, the failure is in the ignition sequence: most often a cracked hot-surface ignitor, a dirty flame sensor, a gas supply issue, or a safety limit doing its job. Furnaces try three times then lock out, which is why the fault seems to come and go when you reset the thermostat.
The ignition sequence, so the symptoms make sense
A modern furnace proves the vent is clear, lights the ignitor until it glows, opens the gas valve, lights the burners, then proves flame with the flame sensor. A failure at any step stops the sequence safely. A furnace that glows but never lights points at gas delivery; one that lights then dies in seconds points at the flame sensor; one that never glows points at the ignitor itself.
Safe checks before you call
Confirm the thermostat is set to heat, the furnace switch (it looks like a light switch, often on or near the unit) is on, the gas valve at the furnace is open, and the filter is not clogged solid, because overheating from a blocked filter also shuts furnaces down. If you smell gas at any point, stop, leave the house and call your gas utility. Beyond those checks, ignition work involves gas and high temperatures and belongs to a professional; see our furnace repair page.
Why ignitors fail so often
Hot-surface ignitors are ceramic and become brittle with years of heat cycles, especially through a College Grove winter of short, frequent runs in mild weather. They are a wear part, not a sign of a bad furnace, and replacing one is among the most routine repairs we do. Our heating repair team carries the common ones on the truck.