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How to Clean a Dirty Furnace Flame Sensor That Shuts Down Gas

Time required: 15-20 mins · Difficulty 1/5

Always shut off the furnace's power switch before opening the burner compartment. If you ever smell gas at any point, stop immediately, leave the area, and call your gas utility or a licensed technician instead of continuing this repair.

Est. DIY Parts: $6 - $10 · Pro Service: $130 - $220 · Est. Savings: $120 - $210

* Estimates based on regional/national contractor rate benchmarks. Actual part prices vary by brand and retailer.

Parts & Tools You'll Need

Step-by-Step Manual

  1. Turn off power to the furnace Flip the furnace's dedicated power switch (usually a light-switch-style toggle nearby, or the breaker) to OFF.
  2. Remove the burner access panel Unscrew or unclip the front panel covering the burner assembly to expose the flame sensor — a thin metal rod near the igniter.
  3. Unscrew and remove the sensor Remove the single mounting screw holding the flame sensor bracket and gently pull the sensor free from the burner assembly, disconnecting its wire.
  4. Clean the sensor rod Lightly rub the metal rod (not the wire connector) with fine steel wool or light-grit emery cloth (roughly 200 to 400 grit, not coarse sandpaper) to remove the thin layer of carbon/soot buildup, then wipe clean with a dry cloth.
  5. Reinstall the sensor Reconnect the wire, screw the sensor back into its bracket in the same position, and replace the access panel.
  6. Restore power and test Turn the furnace back on and set the thermostat to call for heat. Confirm the burners ignite and stay lit (rather than igniting and shutting off after a few seconds).

Frequently Asked Questions

How do I know if my furnace flame sensor is dirty?

A common sign is a furnace that lights, runs for a few seconds, and then shuts off, sometimes repeating before it stays on. The sensor confirms the flame by passing a tiny current through it, and carbon buildup can keep it from detecting the flame. Other problems, such as poor combustion air, can cause similar shutdowns, so call an HVAC pro if cleaning does not help.

What do I use to clean a furnace flame sensor?

Fine steel wool or light-grit emery cloth (roughly 200 to 400 grit) is commonly recommended. Lightly rub only the carbon off the rod, avoid scrubbing away the metal, and wipe it clean. Turn off power to the furnace before you start.

When should I replace a flame sensor instead of cleaning it?

Replace it, or call an HVAC pro, if the ceramic base is cracked or if the furnace still shuts off after a careful cleaning. Many flame sensors do not need replacing unless they are damaged.

Is it safe to clean a flame sensor myself?

Turning off power to the furnace first is essential. This guide covers minor maintenance only. If you ever smell gas, stop, leave the area, and call your gas utility or an HVAC professional.

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