Furnace Not Heating
If a gas furnace won't light or shuts off seconds after lighting, the ignitor or flame sensor is the usual culprit. Cleaning a flame sensor is a simple job; replacing it costs about $80–$250, an ignitor $100–$250, and most furnace repairs $150–$600.
Last reviewed September 2026
Troubleshoot it now
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Check the thermostat, switch and breaker
Thermostat on heat and above room temperature, furnace power switch on, breaker not tripped.
Signs it's this problem
- Furnace runs the blower but blows cold air
- Burners light, then shut off after a few seconds
- Repeated clicking with no flame
- A blinking error code on the furnace control board
Why it happens
A modern gas furnace lights its burners with a hot-surface ignitor, then confirms the flame with a flame sensor. Ignitors crack with age, and flame sensors get coated with soot, so the control board shuts the gas off for safety.
Not sure? Run the heating and cooling problem finderHow to fix it, step by step
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Safety: If you smell gas, don't touch any switches; leave the house and call your gas company. A carbon monoxide alarm means get everyone out and call 911.
Furnace Not Heating cost
| Job | Typical cost |
|---|---|
| Typical furnace repair (average about $350–$400) | $150–$600 |
| Replace a flame sensor | $80–$250 |
| Replace an ignitor | $100–$250 |
| Diagnostic visit | $100–$200 |
Got a quote? Check if it's fair
Typical professional price for this job: $150–$600
Common questions
Why does my furnace turn on then off after a few seconds?
Most often the flame sensor can't detect the flame because it's dirty, so the furnace shuts the gas off for safety. Cleaning or replacing the sensor usually fixes it.
Why is my furnace blowing cold air?
The burners aren't lighting or staying lit (ignitor, flame sensor, gas supply), or the furnace overheated from a clogged filter and shut the burners off while the fan kept running.