What is a flame sensor?
Updated August 2026
A flame sensor is a thin metal rod that sits directly in the burner flame and proves to the furnace control board that the gas actually lit. It does not create the flame; the igniter does that. The rod works on a quirk of physics: a burning flame conducts a tiny current, measured in microamps, from the rod to ground. A healthy furnace reads about 2 to 6 microamps. Drop below roughly 1, and the board assumes there is no flame and closes the gas valve within seconds.
Where it is
Take the lower access panel off a gas furnace and look at the row of burners. At one end, usually opposite the igniter, a single bent rod about 3 inches long pokes into the flame path from a white or tan porcelain base. One wire runs from it back to the control board, and one 1/4-inch hex screw holds it down, so two minutes with a nut driver gets it out. The other part in that neighborhood, a ceramic block with a two-wire plug, is the igniter, not the sensor.
What it does
Everything about this part is safety. If the burners never light, the board has to know within a few seconds so it can shut the valve instead of filling a heat exchanger with raw gas. The catch is that the signal is microscopic, so a film of silica and carbon far too thin to see will insulate the rod enough to break the circuit. The symptom is unmistakable once you know it: the furnace fires, the burners run 3 to 7 seconds, everything shuts down, and the blower purges. The board retries two or three times, then locks out for an hour and blinks a code on the small LED behind the door. That sequence is the most common no-heat service call of the winter, and the repair is often just cleaning a $15 part.
Maintenance
Clean the sensor once a year, in the fall, alongside the filter change and the rest of a heating-season startup. Cut power at the furnace switch and the breaker, back out the screw, and scuff the rod with a folded dollar bill, a Scotch-Brite pad, or 400-grit or finer sandpaper until it is dull gray. Coarse abrasives and steel wool score the metal and make it foul faster. Interval detail is in how often to clean a flame sensor, and the procedure is in how to clean a furnace flame sensor. If short-cycling returns within a month or two, the rod is worn out or the burners themselves are dirty.
What it costs to replace
A replacement flame sensor runs $10–30 for the part itself, or $80–250 for a technician to clean or swap it on a service call.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Is a flame sensor the same as a thermocouple?
They do the same job on different generations of equipment. A thermocouple belongs to older standing-pilot furnaces and water heaters: heat from the pilot generates a small voltage that physically holds the gas valve open, so no pilot means no gas. A flame sensor is the electronic descendant, used on furnaces with hot-surface or spark ignition, and it reports to a control board rather than holding the valve itself. Same principle, different hardware, and the two are not interchangeable.
How do I tell a bad flame sensor from a bad igniter?
Watch one startup cycle. If the igniter glows bright orange, the burners light, and the flame dies a few seconds later, suspect the sensor. If nothing glows and the burners never light at all, the igniter or the control board is the problem. A furnace that lights, stays lit, and still blows cool air is a third story entirely, usually a blower fault or a thermostat left on ON instead of AUTO.
Can a furnace run without a flame sensor?
No, and it should not. Removing or jumping the sensor defeats the one check that closes the gas valve when the burners fail to light, and unburned gas inside a heat exchanger is how furnaces cause explosions and carbon monoxide events. A furnace that only runs with the sensor bypassed still has the original fault. Paying $80–250 for a technician to find it is the right trade.