What a flame sensor does
A flame sensor is a thin metal rod positioned in the burner flame. It uses flame rectification: a flame conducts a very small electrical current, measured in microamps, and the control board watches for it. If the board does not see that current within a few seconds of opening the gas valve, it assumes the burners did not light and closes the valve immediately.
That is exactly the behavior you want. A furnace that kept the gas valve open without confirmed flame would fill the heat exchanger with unburned gas. The sensor is a safety device, and when it acts up it is failing safe.
Why it fails
- Oxidation and carbon deposits build on the rod over months of operation, insulating it from the flame's current.
- Cracked ceramic insulator at the base allows the signal to ground out.
- Poor flame quality from dirty burners means the flame is not properly enveloping the rod.
- A loose or corroded wire connection weakens the signal path.
- Weak equipment grounding, since flame rectification depends on a good ground reference.
How it is diagnosed properly
A meter in series with the sensor reads the actual microamp signal while the burners are lit. Most furnaces need a minimum somewhere around 0.5 to 1.5 microamps depending on the manufacturer, and a healthy system typically reads well above that. A reading of, say, 1.1 microamps on a furnace with a 0.7 minimum tells you the furnace will keep running today and fail on a cold night in three weeks — which is the difference between a measured diagnosis and a guess.
After cleaning to bare metal with the correct abrasive — not sandpaper, which can leave a residue — the signal is measured again to confirm the repair actually worked.
Why we do not recommend DIY cleaning
It requires shutting off gas and power, working inside the combustion compartment, removing a component that is part of the furnace's safety chain, and reinstalling it in the correct position in the flame path. Without measuring the microamp signal afterward, you also have no idea whether you fixed it or just postponed the failure.
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Why Buffalo furnaces do this so often
Oxidation accumulates with run hours and cycle count. A furnace working through a Western New York heating season that stretches from October into April simply puts far more time on the sensor than one in a milder climate. This is also why flame sensor cleaning and microamp measurement are standard parts of annual maintenance here — catching a declining signal in October prevents a no-heat call in January.