Furnace repair

How to Replace a Furnace Flame Sensor

Direct answer: To replace a furnace flame sensor, turn off power to the furnace, remove the burner access panel, swap the old sensor for a matching replacement, reconnect the wire, and test a full heating cycle to make sure the burners stay lit.

A weak or failed flame sensor can let the burners light and then shut back off a few seconds later because the furnace is not proving flame reliably. This is usually a straightforward part swap, but you need to work carefully around sharp metal panels and hot burner parts.

Before you start: Match the sensor rod shape, connector, and furnace compatibility before ordering. Stop if the repair becomes unsafe or unclear.

Last reviewed: 2026-03-25

Step-by-step fix

Step 1: Make sure the flame sensor is a likely cause

  1. Watch what the furnace does during a call for heat if you can do it safely through the sight glass or with the panel in its normal viewing position.
  2. A flame sensor problem usually looks like this: the inducer starts, the igniter glows or sparks, the burners light, and then the flame shuts off again within a few seconds.
  3. Turn the thermostat off after that quick check so the furnace is not trying to restart while you work.
  4. Locate the replacement sensor and compare its rod shape, bracket style, and wire connection to the old one before opening the furnace.

If it works: The furnace behavior matches a flame-proving problem, and the replacement sensor appears to match the original.

If it doesn’t: If the burners never light at all, or the replacement sensor does not match the original, pause and confirm the diagnosis and part before taking the furnace apart.

Stop if:
  • You smell gas before or during the check.
  • The burner area shows heavy rust, loose wiring, scorch marks, or obvious damage.
  • The furnace cabinet or access panel must be forced open to continue.

Step 2: Shut off power and open the burner compartment

  1. Set the thermostat to off.
  2. Turn off power to the furnace at the service switch or breaker.
  3. Wait a few minutes for hot parts near the burners to cool down.
  4. Remove the burner access panel and set the screws aside where they will not get lost.
  5. Use a flashlight to find the flame sensor. It is usually a small metal rod mounted in front of one burner with a single wire attached.

Step 3: Remove the old flame sensor

  1. Pull the wire connector off the flame sensor terminal by gripping the connector, not the wire itself.
  2. Remove the mounting screw that holds the sensor bracket to the burner assembly.
  3. Slide the flame sensor out carefully so you do not bend the bracket or scrape the burner face unnecessarily.
  4. Compare the old sensor to the new one side by side to confirm the mounting tab, rod length, and connector style are the same.

Step 4: Install the new sensor

  1. Place the new flame sensor in the same position and orientation as the old one so the rod sits in the burner flame path.
  2. Reinstall the mounting screw and tighten it snugly without overtightening.
  3. Reconnect the wire terminal firmly to the sensor spade connector.
  4. Check that the wire is routed away from hot burner surfaces and is not pinched by the panel edge.

Step 5: Reassemble the furnace and restore power

  1. Reinstall the burner access panel fully. Many furnaces will not run correctly if the panel is not seated properly.
  2. Turn power back on at the service switch or breaker.
  3. Set the thermostat to call for heat.
  4. Listen and watch through the normal viewing area as the furnace starts its ignition sequence.

If it doesn’t: If the furnace does not start at all, recheck that the access panel is fully in place and the power switch or breaker is on.

Step 6: Confirm the repair holds through a full heat cycle

  1. Let the furnace run long enough to prove the burners stay lit instead of shutting off after a few seconds.
  2. Wait for the circulating blower to come on and check that warm air is reaching the supply vents.
  3. Let the thermostat satisfy normally, then allow the furnace to start again on the next heat call if practical.
  4. Check once more that the burner area stays stable and the access panel remains secure.

If it works: The burners stay lit, the blower delivers warm air, and the furnace completes normal heating cycles without short shutdowns.

If it doesn’t: If the burners still light and shut off quickly, the problem may be elsewhere in the ignition or control system and the furnace needs further diagnosis.

Stop if:
  • The furnace repeatedly locks out, trips the breaker, or shows signs of overheating.
  • You smell gas or see unstable burner flames during operation.

Replacement Parts

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FAQ

Can I clean the flame sensor instead of replacing it?

Sometimes yes. A dirty flame sensor can often be cleaned lightly with fine abrasive material and reinstalled. If cleaning only helps briefly, the sensor is pitted, or you already have the correct replacement, replacing it is the more reliable fix.

What does a bad furnace flame sensor usually look like?

A common symptom is burners that ignite and then shut off again within a few seconds. The furnace may retry several times and then lock out.

Where is the flame sensor in a furnace?

It is usually mounted at the burner assembly with one screw and one wire attached. The metal rod extends into the burner flame so the control board can confirm flame is present.

How long does this repair usually take?

For a straightforward swap, many homeowners can do it in about 15 to 30 minutes once the furnace is cool and the replacement part is on hand.

Do I need to shut off gas to replace the flame sensor?

For this specific part swap, the key safety step is shutting off electrical power to the furnace. If you smell gas or suspect a gas leak at any point, stop immediately and do not continue.

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