A furnace that won't light usually fails in a predictable way, and the cause is often a small, inexpensive part. Knowing how a modern gas furnace starts up makes it much easier to describe the problem to a technician, spot the simple fixes, and judge whether a quote makes sense.
How a gas furnace lights
Most furnaces built in the last 25 years follow the same sequence each time the thermostat calls for heat:
- The control board receives the call for heat.
- The inducer motor starts, pulling air through the burners and pushing exhaust out the flue.
- A pressure switch confirms the venting is working.
- The hot surface igniter glows bright orange (older furnaces use a spark or pilot).
- The gas valve opens and the burners light.
- A flame sensor confirms there's a flame. If it doesn't, the gas valve closes within seconds.
- After a short delay, the blower starts and warm air reaches the house.
Where the sequence stops tells you a lot about which part is failing.
Match the symptom to the likely cause
| What you see or hear | Likely cause | Typical repair cost |
|---|---|---|
| Nothing happens at all | Thermostat, power switch, breaker, blown fuse on the board, or the door safety switch | $0 – $480 |
| Inducer hums or runs, but no glow | Pressure switch, blocked vent or condensate drain, or the igniter | $100 – $375 |
| Igniter glows, but burners never light | Gas supply, gas valve, or control board | $200 – $800 |
| Burners light, then shut off within seconds | Dirty or failing flame sensor | $80 – $250 |
| Clicking, then nothing | Cracked igniter | $100 – $250 |
| Loud rattling or grinding at startup | Inducer motor bearings | $400 – $1,100 |
| Heats briefly, then shuts down; runs again later | Overheating from a dirty filter, or a failing limit switch | $0 – $375 |
Repair costs are typical installed prices from HomeGuide's 2026 furnace repair data. See the furnace repair cost calculator for every part.
Safe checks you can do first
These checks don't involve opening gas or electrical components and solve a surprising number of no-heat calls:
- Thermostat. Make sure it's set to Heat, set several degrees above room temperature, and has fresh batteries if it uses them.
- Furnace power switch. Usually a light switch on or near the furnace. It's easy to turn off by mistake.
- Breaker. Check the furnace breaker in your panel.
- Air filter. A clogged filter can make the furnace overheat and shut itself off. Replace it if it's dirty.
- Front panel. The furnace won't run if the blower door isn't seated, because a safety switch stays open.
- Vents outside. On high-efficiency furnaces, the plastic intake and exhaust pipes can be blocked by snow, ice, leaves or nests.
- Error code. Most control boards have a blinking LED. Count the blinks and compare with the chart inside the furnace door. Write it down for the technician.
Then turn the power off for 30 seconds and back on to reset the board. If it still won't light, call a technician. Cleaning a flame sensor is a common DIY task, but if you aren't comfortable working inside the furnace, leave it to a pro.
What a service visit will likely cost
Most furnace repairs cost $125 to $480, with a national average of about $300. Standard service rates typically run $75 to $150 per hour, and emergency rates $140 to $210 per hour. If the house is safe overnight, a next-morning appointment usually avoids emergency pricing.
Use the furnace repair cost calculator to price a specific part, with or without emergency rates, adjusted for your state.
When a no-heat call becomes a replace decision
Two findings commonly lead to a replacement conversation:
- A cracked heat exchanger. This can let carbon monoxide into the home. Repair typically costs $1,000 to $3,000, and on most older furnaces replacement is the safer choice. Ask to see photos or camera footage, and get a second opinion before spending thousands.
- An expensive repair on an old furnace. Multiply the furnace's age by the repair cost. If the result is over 5,000, get a replacement quote too. Our repair-or-replace guide explains this in more detail.
Preventing a no-heat night
- Schedule a tune-up in early fall, typically $70 to $200. Technicians clean the flame sensor and test the igniter and capacitor, the parts most likely to fail on the first cold night.
- Change the filter every one to three months during heating season.
- Keep outdoor vents clear of snow and debris.
- Install carbon monoxide alarms on every level of the home and near bedrooms.
See our seasonal maintenance checklist for the full fall list.
Frequently asked questions
Why does my furnace light and then shut off?
The most common cause is a dirty or failing flame sensor. The burners light, but the sensor doesn't detect the flame, so the control board shuts the gas off for safety. Cleaning or replacing the sensor typically costs $80 to $250.
How much does it cost to fix a furnace that won't ignite?
It depends on the part. An igniter typically costs $100 to $250, a flame sensor $80 to $250, a pressure or limit switch $100 to $375, a gas valve $200 to $800, and a control board $300 to $650.
Can I reset my furnace?
Turning the furnace power switch off for about 30 seconds and back on resets most control boards. If the furnace locks out again, call a technician rather than resetting it repeatedly.
Is it dangerous if my furnace won't light?
A furnace that won't light is usually safe, because its safety controls are doing their job. It becomes dangerous if you smell gas or a carbon monoxide alarm sounds. In either case, leave the house and call for help.
Sources: HomeGuide, furnace repair costs.