Troubleshooting a gas furnace
The furnace clicks, but it won't light.
Before the ignitor or the flame sensor, there are the simple ones: the thermostat, the power, the filter and the gas valve.
Before you call anyone
No flame: check, understand, then stop
A gas furnace still needs electricity for its blower, its ignition and its control board, so the power checks matter here too.
Safe to check yourself
Check these first
From Trane's advice to owners.
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Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
Worth a visit from us
What it usually means
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The ignitor
Newer furnaces light with an electronic ignitor rather than a standing pilot. If it has failed, the burners never light.
What we do: We test it and replace it if it has failed.
Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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The flame sensor
It shuts the gas off when it can't see a flame. Dirty or faulty, it stops the furnace lighting. Trane says to leave it to a professional.
What we do: We clean or replace it and check the flame it's reading.
Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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Dirty burners
Burners caked with dirt may not light. Trane doesn't recommend opening the furnace to clean them yourself.
What we do: We clean them on a tune-up visit.
Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
Stop, and call
When to stop and call
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The breaker has tripped
Don't reset it. Trane says it most likely tripped because of a problem with the furnace itself.
Source: Trane, Why Is My Furnace Not Igniting?, checked 2026-10-22.
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You smell gas
Don't troubleshoot the furnace. Carrier's advice: if you suspect a leak, leave it alone.
Source: Carrier, Your Guide to a Furnace Not Turning On, checked 2026-10-22.
Tell us what it's doing. We'll take it from there.
Call and describe the problem: the noise, the smell, the room that never warms up. Or book a visit online.
