Furnace will not light: four checks, then call
No heat in cold weather freezes pipes and makes the house unsafe for the old and the very young. The four homeowner checks take five minutes; the parts that fail most, the ignitor and flame sensor, are cheap but not DIY.
What is probably happening
- A failed hot-surface ignitor. It glows, cracks with age, and one day does not.
- A dirty flame sensor. The burners light for a few seconds, then shut off because the sensor cannot see the flame.
- A pressure switch or blocked vent: the inducer fan runs but the burners never get permission to light.
- Or simply the gas valve got turned off, the furnace switch got knocked off, or the thermostat is not calling.
Do this in the next five minutes
- Thermostat: set to HEAT and at least 5 degrees above the room temperature.
- Power: the light-switch style switch on the furnace is ON, and its breaker is not tripped. If tripped, leave it and call.
- Gas: the valve handle on the pipe into the furnace runs in line with the pipe (open), not across it.
- Filter: replace it if it is dirty. A clogged filter trips the high limit.
- Outside: make sure the white PVC intake and exhaust pipes are not blocked by snow, ice or leaves.
- Watch one cycle through the sight glass. Glow but no flame: ignitor or gas. Flame that dies in seconds: flame sensor. Tell the technician which.
Do not
- Do not keep cycling the power to force it. Repeated lockouts fill the cabinet with gas.
- Do not open the burner compartment or clean the flame sensor yourself unless you know the unit. Trane says opening the furnace risks the warranty.
- Do not use the oven or a grill to heat the house. The CDC lists that as a carbon monoxide cause.
Who to call
A licensed HVAC technician
Gas furnace ignition parts and the gas valve are licensed work in most states. The visit should end with a combustion test, not just a flame.
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What it should cost
$150 to $500
Furnace ignitor replacement, national range. Flame sensor $150 to $250. Furnace repairs average $318
Source: Angi, furnace repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Which part failed, and can I see the old one?
- Did you check the pressure switch and the venting, not just swap the ignitor?
- Will you test carbon monoxide at the registers before you leave?
- Is the furnace still under the manufacturer's parts warranty by serial number?
End the call if
- A cracked heat exchanger diagnosis with nothing shown to you, used to sell a whole furnace. Ask to see it on camera.
- An ignitor bill far above $500 with no other work.