Furnace Not Igniting / Blowing Cold Air — Fixes
When the furnace runs the blower but blows cold air, or won't fire at all, several common, homeowner-checkable things are usually to blame before you call for service. Work through these first. Safety first: if you smell gas (a rotten-egg odor) anywhere near the furnace, do not attempt any of the steps below. Leave the house immediately, don't flip switches or use your phone inside, and call your gas utility or 911 from outside. A furnace that repeatedly tries to ignite and fails can also point to a cracked heat exchanger and a carbon monoxide risk -- install a CO detector near sleeping areas and have the unit inspected by a licensed technician if you have any doubt.
1. Check the Thermostat and Filter
Confirm the thermostat is set to HEAT with the temperature above the room reading, and replace the batteries if it's battery-powered. Then check the air filter — a clogged filter restricts airflow, overheats the furnace, and trips a safety limit that shuts off the burner. A dirty filter is a surprisingly common no-heat cause.
2. Dirty Flame Sensor (Very Common)
On modern furnaces, the flame sensor confirms the burner lit. When it's coated in carbon buildup, it can't sense the flame, so the furnace lights for a second then shuts the gas off (you may hear it try repeatedly). Turn off the furnace power AND the gas valve before opening the burner compartment. Then gently clean the flame sensor with fine sandpaper or a scouring pad to fix this extremely common no-heat problem.
3. Check Power and Switches
Make sure the furnace power switch (looks like a light switch near the unit) is on, the breaker isn't tripped, and the furnace door panel is fully seated — a door-interlock switch stops the furnace if the panel is loose. Also confirm the gas is on.
4. Igniter and Condensate
A cracked hot-surface igniter won't light the burner — if it doesn't glow on a call for heat, it likely needs replacing (a common part). Turn off power and the gas valve before accessing it. On high-efficiency furnaces, a clogged condensate drain trips a safety float switch and stops ignition — clear the drain line.
Parts & Tools
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FAQ
Often a dirty flame sensor (the burner lights then shuts off), a clogged filter tripping a safety limit, a thermostat set to "fan on," or an igniter problem. Clean or replace the filter and flame sensor first.
Turn off the furnace power and gas, remove the flame sensor (usually one screw), gently rub the metal rod with fine sandpaper or a scouring pad to remove carbon buildup, and reinstall. This fixes a very common no-heat issue.
Always unplug an appliance and shut off its water supply before servicing. This guide is informational and not a substitute for a qualified technician.