THE SHORT ANSWER
If your furnace lights, then shuts off after a few seconds, the flame sensor is a common cause.
- Try first: a gentle cleaning.
- Replace it if: the rod is bent or worn, or the porcelain is cracked.
- Still failing? call a licensed tech.
Safety first
If you smell gas, do not touch any switch. Leave the house and call your gas company from outside. Before you start, turn off the furnace power switch and turn the gas valve to off. Let the burners cool.
1What a flame sensor does
The flame sensor is a thin metal rod that sits in the burner flame. It proves to the furnace that the gas lit. If the furnace does not get that signal, it shuts off the gas for safety.
2Signs of a dirty or bad sensor
- The burners light, then go out after 3 to 10 seconds.
- The furnace tries a few times, then stops (lockout).
- The rod has a white, black or brown coating.
3How to clean it
- Turn off power and gas. Let it cool.
- Find the rod near the burners. It is held by one screw and has one wire.
- Take a photo, then remove the screw and pull the sensor out. Leave the wire on if you can.
- Rub the rod gently with fine steel wool or a fine abrasive pad. Do not use coarse sandpaper. Do not touch the porcelain with tools.
- Wipe it with a clean dry cloth. Put it back the same way.
- Turn gas and power back on. Run the furnace and watch a full start.
4When to replace it
- The porcelain base is cracked.
- The rod is bent, thin or pitted.
- The furnace still shuts off after a careful cleaning.
To match a new one, compare the rod length, the bracket and the wire connector with your old sensor.
When to call a pro
If a clean or new sensor does not fix it, the cause may be a bad ground, low gas pressure or the control board. A licensed tech can measure the flame signal and find the real problem. Never bypass a flame sensor.
Ready to match the numbers?
Filter by the specs printed on your old part.