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Fix a flame sensor that is dirty

We'll confirm the burner lights then shuts off, locate the flame sensor, clean it, and tell you when to call a pro.

Troubleshooting · Home maintenance
10–25 min
What you'll need
  • Emery cloth or fine steel wool
  • Screwdriver (to remove the flame sensor)
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Steps

Goal: Confirm the burner lights then shuts off, locate the flame sensor, clean it, and know when to call a pro.

  • Set the thermostat to call for heat. Watch the burner.
  • Good: The burner lights but shuts off within a few seconds—flame sensor may be dirty. Proceed to Clean flame sensor.
  • Bad: The igniter does not glow—see Fix an igniter that will not glow. The igniter glows but no flame—gas valve issue; call a pro.

Clean flame sensor

Goal: Remove carbon and oxide buildup from the flame sensor so it can detect the flame.

  • Shut off power and gas. Open the burner compartment per your model. Check your owner’s manual or search “[brand] [model] furnace service manual” for access steps.
  • Locate the flame sensor—a thin metal rod in the path of the burner flame, with a single wire connected.
  • Remove the screw or nut that holds it. Pull it out gently. Clean the metal rod with emery cloth or fine steel wool. Wipe with a clean, dry cloth.
  • Reinstall the flame sensor in the same position. Restore power and gas. Set the thermostat to call for heat.
  • Good: The burner lights and stays lit. The appliance heats.
  • Bad: The burner still shuts off—call an HVAC technician. The flame sensor or control board may have failed.

When to get help

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. Do not call a technician for an active gas leak.

Call an HVAC technician if:

  • The igniter does not glow.
  • The igniter glows but no flame appears (gas valve).
  • The burner still shuts off after cleaning the flame sensor.
  • You are not comfortable working with gas appliances.

Verification

  • The burner lights when the thermostat calls for heat.
  • The burner stays lit and the appliance heats as expected.
  • No error codes or lockouts.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the burner lights but shuts off within a few seconds.
  2. Clean flame sensor Shut off power and gas, remove the flame sensor, clean with emery cloth, reinstall.
  3. Call a pro Gas smell—evacuate, call 911 or gas utility. Burner still shuts off after cleaning—call an HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Appliance type and model
  • Whether the burner lights at all
  • Whether the burner stays lit after cleaning
  • Steps already tried

Does the burner light but then shut off within a few seconds?

The igniter glows, the burner lights, then goes out. A dirty flame sensor often causes this.

Set the thermostat to call for heat. Watch the burner. Good: burner lights then shuts off quickly—flame sensor may be dirty. Bad: igniter does not glow—different problem (igniter guide); igniter glows but no flame—gas valve (call a pro).

Yes No

You can change your answer later.

Clean the flame sensor

Shut off power and gas. Remove the flame sensor. Clean with emery cloth.

Shut off power and gas. Locate the flame sensor—a thin metal rod in the flame path. Remove it. Clean with emery cloth or fine steel wool. Reinstall. Restore power and gas. Retry. Good: burner stays lit—cleaning fixed it. Bad: still shuts off—call a pro.

Yes No

You can change your answer later.

Done

The burner stays lit. The appliance is working.

Done

Call a pro

If you smell gas, evacuate immediately. Call 911 or your gas utility from outside. Call an HVAC technician if: the igniter does not glow; the igniter glows but no flame (gas valve); or the burner still shuts off after cleaning the flame sensor.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a flame sensor be dirty?
Combustion produces carbon and oxides that coat the flame sensor over time. A dirty sensor cannot detect the flame, so the control board shuts off the gas valve for safety. Cleaning restores conductivity.
Can I clean a flame sensor myself?
Yes. Shut off power and gas, locate the flame sensor, remove it, and clean the metal rod with emery cloth or fine steel wool. Reinstall and test. If it still fails, call an HVAC technician.
When should I call a pro for a flame sensor?
If you smell gas, evacuate and call 911 or your gas utility from outside. Call an HVAC technician if the burner still shuts off after cleaning the flame sensor, or if you are not comfortable working with gas appliances.

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