A flame sensor is a thin metal rod in a gas furnace that sits in the burner flame and confirms ignition using a small electrical current, a process called flame rectification. If the control board does not detect flame within a few seconds of the gas valve opening, it closes the valve and shuts the burners off for safety.
Why it matters to you
A dirty flame sensor is one of the most common reasons a furnace lights, runs for a few seconds, then shuts off and repeats the cycle until it locks out.
Practical guidance
Cleaning or replacing the sensor is routine work during a heating tune-up. Repeated lockouts after cleaning can point to a grounding, wiring or control board problem and need a pro's diagnosis.