Flame sensor
A single metal rod set in the burner flame that lets the furnace control confirm a flame is actually burning. It works by flame rectification: the ignited gas conducts a small direct current between the rod and the burner ground, and the board reads that microamp signal as proof of flame.
Also called flame sense rod, flame rectification rod, flame rod
What does a furnace flame sensor do?
A furnace flame sensor is a rod in the burner flame that proves ignition by passing a small rectified current to ground, so the control keeps the gas valve open only while a flame is present.
Flame sensor: how it works
When the igniter lights the burner, the control applies an AC voltage to the flame rod. Flame gas conducts current more readily in one direction than the other because the rod and the burner surface differ greatly in area, so the alternating signal is partly rectified into a small DC current. The board looks for that current within a few seconds of the gas valve opening. If it is absent or too weak, the control closes the valve, waits, and tries again, then locks out after a set number of failed trials.
Where you meet the term
- On a furnace that ignites, burns for two or three seconds, then drops out and retries, sometimes several times before locking out.
- In a fault code that reports a weak or lost flame signal rather than a failed ignition, pointing at the rod, its wire, or the burner ground.
- In a service visit where the rod is lifted out, cleaned back to bare metal, and the microamp reading is measured against the control's stated minimum.
How to check it
- Note how the burner behaves at start-up. Watch whether the flame establishes and then drops after a couple of seconds, and count how many times it retries before the furnace locks out. That pattern is what a technician needs. Owner check
- Measure the flame current in microamps. A technician puts a meter in series with the flame-sense wire and compares the reading with the control's published minimum, then cleans the rod and re-measures before deciding on a part. Technician
Flame sensor: what it is not
- A flame-sense fault is not the same as a bad igniter. The igniter lights the gas; the sensor only confirms it lit, and the burner will be seen to light on this fault.
- Sandpaper is the wrong tool for a coated rod. A technician uses a non-abrasive pad so the rod is not thinned, and a cracked ceramic insulator means the rod is replaced rather than cleaned.
Flame sensor questions
Why does my furnace light then shut off after a few seconds?
The burner is lighting, but the control is not seeing enough flame current to keep the gas valve open. A filmed flame rod, a poor ground, or a damaged sense wire all produce that.
Can I clean the flame sensor myself?
It sits inside the burner compartment behind the furnace door, near the gas valve and line voltage. Identifying and cleaning it without thinning the rod is technician work, not an owner check.
Does a flame-sensor fault mean the sensor is bad?
Not on its own. The reading also drops with a corroded burner ground, a chafed sense wire, low gas pressure, or misaligned burners, so the current is measured and the rod cleaned before anything is replaced.
Where this definition comes from
Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Furnace main burner shuts off after ignitionTrane residential page on a burner that lights and then stops: restricted airflow from a dirty filter overheating the heat exchanger and tripping the limit switch, cracked, rusted or clogged burners, a dirty or faulty flame sensor turning the furnace off early, and control board failure; with the flame sensor described as ensuring the gas valve only opens while flames are burning so unburned gas cannot build up.
- Gas furnace technical frequently asked questionsRheem gas furnace technical FAQ: improper phasing or polarity and a dirty flame sense rod both preventing flame rectification, incorrect manifold gas pressure as a cause of failed flame sense, four ignition trials with a one-blink code reported after four failed trials or four flame failures in one call for heat, reset by removing the 115 volt supply or switching the thermostat off for five to ten seconds, the main limit opening on low airflow across the heat exchanger or excessive manifold gas pressure, and a pressure switch failing to close on a blocked vent, a damaged heat exchanger, or an induced draft blower not running to speed.