A furnace flame sensor is a small metal rod that detects whether the burner is lit by allowing electrical current to flow back to the control board. When oxidation builds up on the sensor, burners ignite then shut off within seconds, triggering a safety lockout. Clean the sensing rod with fine steel wool, wipe it down, reinstall it, and test the heating cycle.
Hey, are you new to the trades? Here's something you may not know about the furnace flame sensor. A flame sensor is a small metal rod that sits where the burner flame can touch it. It does not light the flame. Its job is to send a tiny electrical current through that flame back to the control board. That current tells the board the burner's really lit. Here is a common field symptom: The burners light, then shut off a few seconds later. Often, the sensor has a light coating of oxidation. It cannot pass enough current, so the board closes the gas valve and may eventually go into a safety lockout. The board has to see the flame. With power off, and after following your equipment procedure, remove the sensor carefully. Clean only the metal sensing rod with fine steel wool or emery cloth. Do not use sandpaper or harsh abrasives. They can damage the surface. Wipe away residue, reinstall it securely, and test the full heating cycle. Clean it gently. Don't sand it. A small part can stop the whole furnace for a good reason. Now that you know, you can help someone else know.
