The flame sensor is a grounded metal rod mounted in your furnace’s burner box that confirms ignition actually occurred. When the burner lights, the flame conducts a tiny current—2–4 microamps (µA) DC—from the rod to ground through the flame itself. The control board reads this current; if it’s above the threshold (~0.5 µA on most boards), the gas valve stays open. If it’s below—because the sensor is dirty, oxidized, or the flame didn’t light—the board closes the gas valve within 2–5 seconds to prevent an uncontrolled gas release.
That shutdown cycle, repeated 2–3 times, is what causes a furnace to “try to ignite, then shut off” over and over.
How flame rectification works
Natural gas flames are conductive—they contain ions (electrically charged particles) from the combustion reaction. A flame sensor exploits this:
- The control board applies a small AC voltage to the sensor rod (typically 120V AC at very low current)
- Ions in the flame allow current to pass from the rod to ground
- The asymmetry of the rod-to-ground geometry rectifies the AC signal into DC
- The board measures the resulting DC microamp signal
- If ≥ threshold: flame confirmed, gas valve stays open
- If < threshold: gas valve closes within 2–5 seconds
A dirty sensor has a thin oxide or carbon layer on the rod surface that increases resistance and reduces the current signal below the detection threshold—even when the flame is burning normally.
Symptoms of a bad or dirty flame sensor
- Furnace ignites (you hear the click of the ignitor and see it glow), then shuts off 2–5 seconds later
- Furnace attempts ignition 2–3 times before locking out
- Control board error code: often 3 flashes (varies by manufacturer — check the label inside the furnace door)
- Furnace runs fine in summer AC mode but fails in winter (winter = more starts = faster sensor fouling)
How to clean the flame sensor
Cleaning takes about 10 minutes and is the first thing to try before replacing.
Safety: Turn off power to the furnace at the breaker before proceeding. Gas is controlled by the valve, not by touching the sensor, but always work on a powered-off furnace. Do not use abrasive compounds, sandpaper coarser than 400-grit, or steel wool—these remove the oxide protective layer and accelerate re-fouling.
- Turn off the furnace breaker
- Locate the flame sensor — a 1–3 inch metal rod on a white ceramic mounting bracket, with a single wire lead, positioned in the burner flame pattern
- Remove the one mounting screw (usually 1/4″ hex) and pull the sensor out
- Gently rub the metal rod with a folded piece of #400 fine steel wool or a dollar bill (mildly abrasive) until the rod is shiny
- Reinstall the sensor, restore power, and test
If cleaning doesn’t resolve the issue within 1–2 cycles, measure the flame signal: connect a multimeter in DC microamp mode in series with the sensor wire and read during a heating cycle. Under 0.5 µA means replace the sensor; 2+ µA means the sensor is fine and the issue is elsewhere.
When to replace vs. clean
| Condition | Action |
|---|---|
| Sensor rod is discolored but intact | Clean first |
| Reading is < 0.5 µA after cleaning | Replace sensor |
| Sensor rod is cracked or broken | Replace immediately |
| Problem recurs within a few months after cleaning | Replace sensor; also check for combustion air issues causing heavy carbon |
Replacement sensors typically cost $10–20 and are universal-fit (match the connector type and rod length). This is a straightforward DIY repair with the furnace power off.
How it connects to your thermostat
From the thermostat’s perspective: a W-terminal call for heat should produce a heating cycle that runs until the setpoint is reached. When a flame sensor fails, the furnace starts, then cuts out after a few seconds. The thermostat sees the call as unsatisfied and tries again—leading to short cycling that looks like a thermostat problem but is actually a combustion failure.
If your thermostat is calling for heat continuously and the house isn’t warming up, listen at the furnace: if you hear repeated ignition attempts followed by silence, the flame sensor is the first place to look.
Related glossary terms
- Hot surface ignitor — the component that lights the burner before the flame sensor takes over
- Furnace limit switch — another safety that causes shutdown; causes overheating rather than ignition failure
- Short cycling — the pattern a failed flame sensor produces
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FAQ
Q: My furnace ignites, runs for 2 seconds, then shuts off. Is that the flame sensor? That’s the classic flame sensor symptom. The ignitor lights the burner, the flame comes up, but the sensor doesn’t confirm it—so the board closes the gas valve within 2–5 seconds. Clean the sensor first; if that doesn’t fix it, measure the microamp signal to confirm.
Q: How often should I clean the flame sensor? Every 2–3 years as part of annual furnace maintenance, or whenever you see the “ignites and shuts off” cycle start. Cleaning is a 10-minute job and prevents lockout at the worst possible time.
Q: My furnace runs fine for the first few days of winter, then starts failing. Why? Most likely the sensor is borderline—reading just above threshold when new and clean each season, then fouling faster once the furnace runs regularly. Clean or replace the sensor before winter, not after the first failure.
Q: Can a bad ignitor look like a flame sensor problem? Yes, but the symptom is slightly different. A bad ignitor means the burner never lights at all—you hear the ignitor energize but see no flame. A bad flame sensor means the flame lights briefly, then cuts off. Listen carefully to which happens first.
