How a flame sensor works
A flame sensor is a single metal rod positioned so the burner flame envelops its tip. It does not measure temperature. It relies on flame rectification: an ionized flame conducts a tiny direct current, usually a few microamps, between the rod and the burner ground. The control board watches for that current.
If the board sees the expected microamp signal within a few seconds of opening the gas valve, the cycle continues. If it does not, the board closes the gas valve immediately. That single behavior is what prevents unburned gas from filling a heat exchanger and a home.
Why sensors fail
Buildup is by far the most common. It happens gradually, which is why the symptom often starts as an occasional lockout and becomes a daily problem.
- Carbon or oxide buildup on the rod, which insulates it and reduces the signal
- A cracked ceramic insulator allowing the signal to leak to ground
- A loose or corroded ground connection at the burner assembly
- A rod that has shifted out of the flame path
- Simple age — the rod eventually degrades from constant thermal cycling
Symptoms of a failing flame sensor
The classic pattern is a furnace that lights normally, runs for three to seven seconds, then shuts the burners down and purges. It may retry two or three times before locking out entirely. Homeowners usually describe it as the furnace blowing cold air or the furnace not heating, because the blower still moves room-temperature air.
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What service involves
A technician measures the actual flame signal in microamps with a meter, compares it to the manufacturer's minimum, cleans or replaces the rod, verifies the burner ground, and re-measures. Measuring matters: a sensor that is marginal today will fail on the coldest night of the year, and only a reading shows the difference between clean and barely passing.
This is generally the least expensive category of furnace repair and is usually completed in one visit. It is also part of a thorough furnace tune-up, which is why flame sensor failures are far less common on systems that receive annual service.
Why this is not a DIY job
Accessing the flame sensor means opening the burner compartment of a gas appliance. Beyond the obvious gas and electrical exposure, an improperly reinstalled sensor, a disturbed burner or a compromised ground can create a furnace that appears to run while failing to prove flame correctly. See furnace safety for the controls that should never be modified or bypassed.
Furnace Safety Comes First
If you smell gas, see smoke or fire, or suspect carbon monoxide, stop troubleshooting. Move everyone out of the affected area, then contact your gas utility or emergency services from a safe location.
Never bypass a pressure switch, limit switch, rollout switch, door switch or flame sensor, and never attempt gas valve, burner, heat exchanger or live-voltage work. Those controls exist to shut the furnace down before a dangerous condition develops.
Read our carbon monoxide safety guide