Updated Aug. 13, 2018
The flame sensor is a safety device designed to prevent the unnecessary flow of gas from the furnace unit. It's essentially a confirmation mechanism. When a furnace's main burners ignite, the sensor picks up the heat and signals the main furnace control board that it's ok to allow that main burner to stay on. In the event the main burners don't light, however, the sensor signals the board that there's no need for the gas valve to be open, thus eliminating unnecessary gas from being released into a home's air supply.
Types Of Flame Sensors
Differences are few among flame sensors. Most look like straight pencils, though some--such as ProTech models--are L shaped. Beyond that, the only fundamental differences are lengths of the sensor itself and the porcelain housing.
Replacing A Flame Sensor
The actual replacement of the flame sensor is very easy, as it's just a rod held in place by a screw or two. However, it's important to make sure that what you need is to replace the sensor rather than simply to clean it.
Consider this scenario: The main burner ignites, stays on for about 8 seconds, and then shuts off. The ignition sequence is attempted again, and the failure repeats. Upon a third attempt, the furnace locks down, in which case the only way to get the system out of lockout is to flip the power switch off and on. Everything restarts and the burners remain unlit. It would be easy to view this as an ignition problem, but it has nothing to do with the igniter.
Check the sensor to see if it's cracked (usually occurring on the porcelain portion). If that's not the problem, clean the sensor. Experts agree that in an overwhelming majority of cases, this will do the trick.
Over time, sensors can become engulfed in a light coating of burned-in dust and dirt. When these particles are burned into the sensor, they produce a coating that prevents the sensor from accurately gauging the heat of the flame. With the power to the furnace turned off, sensors may be removed and cleaned with a light sand cloth, emery cloth or steel wool.
Who Makes Flame Sensors?
Many companies make flame sensors. A nonexhaustive list includes: