A flame sensor is a safety feature and part of the electronic ignition system on your gas furnace.
During the ignition cycle, the furnace goes through a sequence of operation where either a spark or a hot surface igniter is used to ignite the gas. When the gas is ignited, the flame sensor creates a current of electricity measured in micro amps. If the main control board does not read the proper level of micro amps, the furnace will discontinue feeding the unit fuel, preventing an explosion.
Over time, oxidation or carbon buildup can interfere with its ability to sense the flame, which can cause the main control board to lock out. The way we determine if a dirty flame sensor is to blame for the lockout is to take a micro amp draw reading, then clean the sensor with steel wool. If dirt was the only factor, we will see a much higher amp reading. If the amp draw is unchanged, the technician will have to continue the diagnostic process.