Flame Sensor
Updated
The flame sensor is a thin metal rod that sits in the burner flame inside a gas furnace and confirms the flame actually lit. If it does not detect flame within a few seconds of the gas valve opening, the furnace shuts the gas off.
That is exactly what you want, since unburned gas has no business accumulating in a furnace. The catch: a sensor coated in residue reads a healthy flame as no flame, so the furnace lights, runs a few seconds, and shuts down over and over. A dirty flame sensor is one of the most common no-heat calls, and the fix is usually a careful cleaning.
Common Questions
What does a flame sensor do?
The flame sensor proves to the gas furnace that its burners actually lit. Once the gas valve opens, the sensor has a few seconds to detect flame; if it does not, the furnace closes the valve so unburned gas never accumulates in the cabinet.
Why does my furnace light and then shut off after a few seconds?
A residue-coated flame sensor is the most frequent cause. The flame is fine, but the sensor cannot see it, so the furnace shuts the gas off as a precaution and tries again. A furnace repair visit confirms whether cleaning the sensor settles it or something else is interrupting ignition.
Can a flame sensor be cleaned instead of replaced?
Most of the time a careful cleaning restores a flame sensor completely, since the rod itself rarely fails. It just builds up residue that blocks its reading. Sensor cleaning is part of a Clean and Check℠ visit, which is how the problem gets caught before the first cold morning.
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