Furnace Starts Then Shuts Off After a Few Seconds? Here's Why
You hear it fire up. Flame for a few seconds. Then it dies — and tries again a minute later, and again. This is different from a furnace that runs a few minutes and quits (that's short cycling). Seconds means the furnace lit and then decided the flame wasn't safe.
Safety first: if you smell gas, stop. Don't flip switches. Leave the house and call your gas company from outside.
What's happening
A modern furnace has a flame sensor — a small rod in the flame. If it can't "see" the flame within a few seconds of lighting, it shuts the gas off to keep unburned gas from piling up. A dirty sensor, a weak flame, or a venting problem all look the same to it: no flame confirmed, shut down.
After a few tries, most furnaces lock out for an hour or until reset.
1. Check the vents outside
High-efficiency furnaces breathe through white PVC pipes on the side of the house. Blocked intake or exhaust — snow, ice, leaves, a nest — makes a weak, unstable flame. Clear them. Never cap them.
2. The filter
Starved airflow overheats the furnace. It can light, hit a limit, and shut down in seconds. Hold the filter to the light. Can't see through it? Replace it and wait 15 minutes before trying again.
3. Reset once
Turn the furnace's power switch (looks like a light switch, on or near the furnace) off for 30 seconds, then on. Once. If it lights and dies again, stop. Repeated resets don't fix a flame-sensing problem; they just cycle gas.
What it usually is
Most "lights then dies in seconds" calls end with a technician cleaning or replacing the flame sensor — a quick, inexpensive fix. Sometimes it's a pressure switch or a weak igniter. All of it is inside the furnace, and none of it is a homeowner job.
The win is walking in with the right story. Run the free troubleshooter: it captures the pattern (lights, runs how long, how many tries), rules out the easy stuff, and gives you a summary to read to the dispatcher.
Related symptoms
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