Modern gas appliances, particularly gas furnaces, employ electronic igniters rather than standing pilots (which are common on older appliances), so they don't need thermocouples to keep the pilot lit. They still need a safety sensor to ensure that gas ignites when the appliance switches on, and this function is served by the flame sensor. Although designs vary somewhat, a typical flame sensor is a 2- to 4-inch metal rod with a porcelain base and is connected to the gas valve by a wire. It's mounted on the burner housing in such a way that the metal rod is in contact with the flame when the gas is burning.
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The job of the flame sensor is to send a small electric signal to the gas valve when the gas ignites to keep the valve open, and when the sensor malfunctions, the valve will close, and the furnace will shut down. This prevents unburned gas from circulating through the air ducts and igniting elsewhere in the house. You can usually identify a defective flame sensor just by looking at it, but if that isn't conclusive, you can also test it with a multimeter.
Symptoms of a Bad Flame Sensor
If the flame sensor for a furnace isn't doing its job, you'll hear the blower switch on, the spark igniter begin to click and the burners ignite when the thermostat calls for heat, but all that activity will stop suddenly, and the furnace will shut down. The cycle will repeat again after a few minutes, but after a few tries, the furnace may go into lockdown mode and need to be reset, which you can usually do by turning off the breaker, waiting a few seconds and turning it back on.
A visual inspection of the flame sensor, which you can locate using your furnace manual, may reveal that the porcelain base is charred. That's usually a sign that you need to replace it. Fortunately, flame sensors are inexpensive parts and are easy to replace. If the base isn't charred, however, there's a good chance you can restore the furnace operation by cleaning the metal probe.
How to Clean a Flame Sensor
On most furnaces, the flame sensor is held to the burner housing by a single screw that you can remove with a Phillips screwdriver. Before you do this, be sure to switch off power to the furnace by turning off the breaker. Pull the sensor out of the cavity and check the condition of the metal rod. If it's blackened by soot, that's probably why it's malfunctioning.
You can clean off the soot with fine-grit emery cloth. Avoid coarse-grit sandpaper or steel wool, which can damage the fragile probe. Simply rub the emery cloth over the probe until the soot is gone, and the metal is more or less shiny. Replace the flame sensor, drive the screw to hold it and turn on the power. If the furnace continues to shut down, you can conduct a more definitive test with a multimeter.
How to Test a Flame Sensor
To determine whether the flame sensor is sending a signal to the gas valve, you have to connect a multimeter than can measure microamps (µA) in series between the sensor terminal and the wire that leads to the gas valve. This test must be conducted with the power on because you need to take the reading when the furnace starts up.
Remove the lead from the flame sensor and connect it to one probe from the meter. Then, connect the other meter probe to the sensor terminal. Turn up the thermostat to call for heat and check the meter reading when the burners ignite. It should read between 5 and 10 µA. If it reads less than this, the sensor is faulty and needs to be replaced.