Components
Flame sensor
A small metal rod that confirms the burner has lit so the gas valve stays open.
The flame sensor is a thin rod that sits in the burner flame. When flame contacts the rod, it generates a tiny current (microamps) confirming combustion. No flame, no current, gas valve closes after a few seconds — safety feature.
Over time the rod gets coated in carbon. The current drops below the threshold, the valve closes prematurely, and you get short-cycling: burner lights, runs 30 seconds, shuts off, retries. Cleaning takes 10 minutes with steel wool. We check it on every tune-up.
If your furnace is short-cycling and you've replaced the filter, the flame sensor is the next thing to check.
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