Furnace Flame Sensor: What It Does, How to Clean It, When to Replace It

If your furnace ignites briefly — you see the flame, feel a puff of warm air — then shuts off and tries again repeatedly, the flame sensor is almost always the culprit. It's one of the most common furnace service calls, and often one of the easiest DIY fixes. Here's everything you need to know.

What Does the Flame Sensor Do?

The flame sensor is a safety component that verifies combustion is actually occurring before allowing the gas valve to stay open. Without it, a failed ignitor would allow the gas valve to remain open indefinitely — filling the heat exchanger with unburned gas, a serious explosion risk.

The sensor works through flame rectification: a small AC voltage is applied to the sensor rod. When the rod is in a flame, ionized combustion gases allow a small DC current to flow back to the control board. The board reads this microamp signal and confirms "flame present — keep the gas valve open." No signal = control board closes the gas valve within 1–3 seconds as a safety shutoff.

What Causes Flame Sensor Failure

The sensor doesn't usually fail catastrophically — it degrades. The ceramic rod develops an oxide coating over its surface through normal heating and cooling cycles. This oxidation layer insulates the rod, reducing the current it can pass through the flame. When the current drops below the control board's threshold (typically 0.5–1.0 microamps), the board interprets it as "no flame" and shuts down — even though combustion is occurring normally.

This is why the symptom is "furnace lights then immediately shuts off" — the burner fires, but the degraded flame sensor can't confirm it, so the board shuts down within 1–3 seconds.

Diagnosing a Flame Sensor Issue

The classic symptom: furnace goes through the normal startup sequence (inducer runs, ignitor glows, burner lights), then shuts off 1–3 seconds after ignition. This repeats 3 times, then the furnace goes into lockout (error code — often 2 blinks on Goodman).

Confirm it's the flame sensor (not the ignitor, gas valve, or control board) by checking:

• Does the burner light? If yes, the ignitor and gas valve are working. The problem is flame sensing.
• Does it shut off within 3 seconds every time? Classic flame sensor degradation.
• If you have a multimeter: measure microamp DC current through the flame sensor circuit during operation. Spec is usually 2–10 microamps; under 0.5 indicates a problem.

How to Clean the Flame Sensor: Step-by-Step

This is a simple 15-minute DIY repair that resolves most flame sensor issues. No special tools required beyond a screwdriver and steel wool.

1. Turn off power to the furnace at the switch or breaker
2. Turn off gas at the shutoff near the furnace
3. Locate the flame sensor — it's a single rod (usually about 2" long) mounted in the burner flame path, connected by a single wire. In most furnaces it's near the end of the burner rack, identifiable by its single wire connector.
4. Remove the single mounting screw and gently pull the sensor out
5. Lightly clean the rod with fine steel wool or emery cloth (220-grit). Rub gently — you're removing the oxide layer, not material. The rod should have a slight metallic sheen when done. Do not use sandpaper (too aggressive) or cleaning chemicals (leave residue).
6. Reinstall — snug the mounting screw, reconnect the wire
7. Restore power and gas, then cycle the thermostat to trigger a heat call
8. Observe: burner should light and stay on continuously through the call for heat

When Cleaning Isn't Enough: Replacement

If cleaning doesn't resolve the problem, or if the ceramic insulator on the sensor rod is cracked, the sensor needs replacement. Replacement flame sensors cost $10–$30 and are available at HVAC supply houses and online. Verify compatibility with your furnace model number.

Installation is identical to the cleaning process — remove one screw, unplug the connector, install the new sensor, reconnect.

Other Causes of "Lights Then Shuts Off" Symptoms

If cleaning/replacing the flame sensor doesn't solve the problem, the issue may be:

• Weak gas pressure: Low manifold pressure produces a small, weak flame that doesn't fully envelope the sensor rod. Requires tech with manometer.
• Incorrect sensor position: Sensor rod not in the flame path. Check that it's positioned inside where the flame burns, not above it.
• Grounding issue: The flame rectification circuit requires a solid electrical ground. A loose ground wire on the furnace chassis can cause intermittent false "no flame" signals.
• Control board failure: The board's flame sensing circuit may be faulty. This is a less common diagnosis but possible on older boards.

Flame Sensor Maintenance Schedule

As part of your annual DIY tune-up, clean the flame sensor every 2–3 years regardless of symptoms. This prevents degradation-induced shutoffs before they happen, especially important heading into a Minnesota winter when a furnace failure has real consequences.

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