How to test and clean a flame sensor
- Takes about 10 minutes
- Beginner-friendly
- 3 tools needed
- Updated
- Reviewed by a licensed tech
A furnace that lights, runs for five seconds and shuts down is almost always a dirty flame sensor — not a broken one. This is the ten-minute fix that costs nothing, and you should try it before you buy anything.
Safety
Power off at the switch and the breaker before the door comes off. The sensor sits in the burner flame path — give it ten minutes to cool before you unscrew it, or you will find out how hot 1,800°F steel stays.
What you'll need
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¼ in nut driver
One screw holds the sensor.
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Scotch-Brite pad, emery cloth, or a folded dollar bill
Anything mildly abrasive and NON-metallic. Not sandpaper, not steel wool — see step 3.
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Multimeter that reads microamps (µA DC)
Optional, and how a pro confirms the fix rather than hoping.
The fix, step by step
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Confirm the symptom: lights, then dies in about five seconds
Watch a cycle. Burners light, run for roughly five seconds, then cut out and the inducer keeps running — that is the board proving flame, failing to see it, and shutting the gas valve. Three tries and it locks out. If it never lights at all, this is the wrong guide; start with the igniter.
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Pull the sensor
It is a single steel rod with a porcelain base, on its own at the far end of the burner assembly from the igniter, with one wire. One ¼ in screw. Slide it out of the flame path — the wire's spade will usually reach far enough that you do not need to unplug it.
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Scuff it — do not polish it
The rod is coated in a dull white oxide film that blocks the microamp signal. Scuff it back to dull grey metal with a Scotch-Brite pad, emery cloth or a folded banknote. Do NOT use sandpaper or steel wool: both leave the surface scratched and embedded with grit, which gives the film more to hold onto and puts you back here in three months.
Safety: Do not touch the porcelain insulator with the abrasive. A crack there sends the signal to ground and no amount of cleaning will bring the furnace back.
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Refit and watch a full cycle
Back in, screw snug, door on. The furnace should light and STAY lit. If you have a meter that reads microamps, break the sensor wire and put the meter in series with it: a clean sensor in a good flame reads 4–10 µA on most furnaces, and anything under about 1.5 µA is why the board is not convinced.
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Still dying? Now it might be the sensor — or it might be the flame
A cleaned rod reading under 1.5 µA in a healthy flame is genuinely failed and worth replacing. But a WEAK, lazy, yellow flame gives a low reading through a perfect sensor, and that is a combustion problem: dirty burners, a blocked flue, or a gas pressure issue. Replacing the rod will not fix it, and the flue is not a DIY diagnosis — that one is a phone call.