Troubleshooting a gas furnace
The heat is on. The air is cold.
Most of the time it's a setting. When it isn't, the furnace is often protecting itself, and the reason matters.
Before you call anyone
Cold air from the vents: check, understand, then stop
Work down in order. Blue is safe for you to check; red is where you turn it off and call.
Safe to check yourself
Check these first
From Carrier's advice to owners. Tick each one off as you go.
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Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
Worth a visit from us
What it usually means
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A dirty flame sensor
The burners light, then shut off, and the fan keeps blowing cold. A dirty sensor can't see the flame, so the furnace cuts the gas as a precaution.
What we do: We clean or replace the sensor, then watch it through a full heating cycle.
Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Leaky ducts
Warm air escapes into the attic or crawlspace before it reaches the rooms, and what arrives is lukewarm.
What we do: We find the leaks and seal them.
Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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The control board
If the board fails, the blower can run without the heat ever coming on. Carrier says not to repair it yourself.
What we do: We read the board's fault code and test it before replacing anything.
Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
Stop, and call
When to stop and call
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You smell gas, or see soot
Turn the system off and get help. If it's gas, leave the house first.
Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, checked 2026-10-22.
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Gas valves and ignition
Carrier warns that DIY fixes on gas parts can be a safety hazard. That part is ours.
Source: Carrier, Reasons Your Furnace Is Blowing Cold Air, 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.
