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How do you clean a furnace flame sensor?

Updated August 2026

20–30 minutes · Easy DIY

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A furnace that lights, runs four to seven seconds, then shuts off almost always has a dirty flame sensor, and cleaning it is a 20 to 30 minute job with a nut driver and fine steel wool. Soot on the rod kills the tiny current that proves a flame exists, so the control board closes the gas valve. No gas fitting comes apart for this. Put it on the yearly list beside the filter change on your home maintenance checklist.

What you need

Before you start: This is gas equipment, so treat it that way. Kill power at the furnace service switch and at the breaker, and give the burner compartment 10 to 15 minutes to cool, because the sensor sits in the flame path and runs well past 1,000°F. Leave the manifold, the gas valve, and every union alone. If you smell gas, leave the house and call the gas utility from outside. Check the filter while you are there, since starved airflow trips the high-limit switch and looks identical from the hallway.

Steps

  1. Turn the furnace off at the thermostat, then cut power at the service switch on the furnace and at the breaker. Confirm the blower and inducer are dead before you open anything.
  2. Let the burner compartment cool 10 to 15 minutes. The rod glows red during a cycle.
  3. Pull the burner access door and find the sensor: one bare metal rod about 3 inches long at the far end of the burner row, with a single wire on it. The igniter looks similar but is ceramic and carries a two-wire plug.
  4. Back out the 1/4 inch hex screw on the bracket, lift the rod free, and slide the connector off its spade terminal.
  5. Scuff the rod with fine steel wool or an emery cloth until the gray film is gone and the metal is bright. Ten to fifteen seconds of light pressure does it. Coarse sandpaper scores the rod and leaves grit that insulates it again.
  6. Wipe the white porcelain insulator at the base with a dry rag. Soot there bleeds the signal to ground and mimics a dirty rod.
  7. Reinstall the rod, push the connector back on, close the burner door so the interlock switch is depressed, and restore power.
  8. Run a heat cycle. The burners should light and stay lit well past 10 seconds. If they still drop out at four to seven seconds, the rod is worn out and a replacement is $10 to $20.

Or hire it out

A service call to clean the sensor runs $80 to $200, and most $100 to $200 annual furnace tune-ups already include it. The DIY version costs a $6 pack of steel wool and half an hour, best spent before the first cold snap rather than during it.

Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for flame sensor cleanings sets itself.

Log this task, get the next reminder

Frequently asked questions

How do I tell a dirty flame sensor from a bad igniter?

Watch one cycle. If the igniter glows, the burners light, and everything shuts down four to seven seconds later, the sensor is failing to prove the flame and the board is correctly closing the gas valve. If nothing glows and no flame appears, suspect the igniter instead. A sensor fault repeats on a loop, usually three tries before the board locks out for an hour. What a flame sensor is explains the proving circuit behind that behavior.

Can I use sandpaper, a wire brush, or a dollar bill?

A folded dollar bill or a paper towel handles light film and cannot damage the rod, which is why many techs carry nothing else. Fine steel wool in 0000 grade or an emery cloth works better on a stubborn gray coating. Avoid coarse sandpaper, a rotary tool, or a steel wire brush: they thin the metal, leave abrasive residue, and roughen the surface so soot grips faster next season.

How often does a flame sensor need cleaning?

Once a year for most furnaces, timed to the start of heating season alongside the filter change. Dusty houses, home workshops, and furnaces in garages foul faster and often want a mid-winter wipe. The rod itself lasts 5 to 10 years before the metal degrades past what cleaning can fix, and then the $10 to $20 replacement is the answer. How often to clean a furnace flame sensor breaks the interval down by situation.

It is clean and the furnace still shuts off. Now what?

Check three things before calling anyone. Reseat the sensor wire and tighten the ground strap between the burner assembly and the cabinet, because flame rectification needs a solid ground path. Fit a new rod if the old one is pitted. Then look at the filter and the registers, since a starved return overheats the exchanger and trips the limit switch. Past that, a tech with a micro-amp meter settles it in ten minutes.

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