Furnace Flame Sensor
If your gas furnace lights, runs for a few seconds, then shuts off and tries again, a dirty flame sensor is the most common cause. Cleaning it takes about 15 minutes with fine steel wool or an emery pad. A replacement sensor costs $10 to $40, and a technician typically charges $75 to $250.
Typical pro cost
$75 to $250
- Urgency
- Fix soon
- DIY difficulty
- Easy
- Time to fix
- 15 to 30 minutes
- Prices
- US typical range
Last reviewed September 2026
Signs it's this problem
- Burners light, then shut off after a few seconds
- Furnace cycles on and off repeatedly
- Error code or blinking light for flame failure
- House never reaches the set temperature
Common causes
Carbon buildup
A thin coating on the sensor rod stops it from detecting the flame.
Cracked sensor
The ceramic insulator can crack with age.
Poor ground or wiring
A loose wire or bad ground weakens the flame signal.
Weak or lifting flame
Burner problems keep the flame from touching the sensor.
How to fix it, step by step
- 1
Turn off the furnace at the thermostat and the power switch, and turn the gas valve off.
- 2
Remove the front panel and find the flame sensor: a thin metal rod in front of the burners with one wire.
- 3
Remove the single screw and gently pull out the sensor.
- 4
Lightly rub the metal rod with fine steel wool or an emery pad until it is shiny. Do not touch the porcelain.
- 5
Reinstall the sensor, reconnect the wire, replace the panel, and restore gas and power.
- 6
Run the furnace. If it still shuts off, call a technician to check the sensor signal, ground and burners.
Tools and materials
- 1/4-inch nut driver or screwdriver
- Fine steel wool or emery pad
- Flashlight
Approximate DIY cost: $0–$40
Call a pro when
- Cleaning did not fix it
- You smell gas
- The burner flame looks yellow or lifts off the burner
Furnace Flame Sensor cost
| Job | Typical cost |
|---|---|
| Technician: clean flame sensor | $75–$200 |
| Replace flame sensor, installed | $100–$250 |
| Furnace service call | $80–$200 |
| Replace hot surface igniter, installed | $150–$400 |
Ranges are typical US prices compiled by FixHomeCost. Prices vary by region, home and contractor, so compare 2–3 local quotes.
Common questions
How do I know if my flame sensor is bad?
The furnace lights but shuts off within a few seconds, often trying three times before locking out. Cleaning the sensor fixes this in most cases.
Can I clean a furnace flame sensor myself?
Yes, it is one of the simplest furnace fixes. Turn off power and gas first and handle the sensor gently.
How often should a flame sensor be cleaned?
Usually during yearly furnace maintenance. Homes with dusty air may need it more often.
What is the difference between a flame sensor and an igniter?
The igniter lights the gas. The flame sensor proves the flame is on so the gas valve stays open.
Related guides
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