A gas furnace flame sensor is part of an electric ignition system that is connected to the gas valve to control the flow of gas to the furnace. A flame sensor can come in both L or straight shapes, and it is part of the furnace burner's basic assembly.
A flame sensor serves a similar function as a thermocouple in a furnace. The key difference between the two components is that a thermocouple is typically only found in furnaces that use a standing pilot light. If a flame sensor is damaged, it is important to wait for the furnace to cool down before replacing it with a new sensor.