How to Fix a Bad Furnace Flame Sensor
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A furnace with a bad furnace flame sensor can be very frustrating. If it can’t detect a flame, it shuts down and stays off for an hour to allow the gas to vent before trying again. This cycle can happen two or three times before the system shuts down permanently. Typically, this is caused by dirt or carbon buildup on the flame sensor.

Where is furnace flame sensor?

In this article we will show you how to locate your burner flame sensor, clean it and test it to see if it works. We recommend that you call an experienced technician to do this work for you as it can be dangerous. They can open your unit, locate the sensor and determine if it is the cause of your short cycles by testing it for proper function.

To test the flame sensor, first turn off your heater at the thermostat and shut off your gas valve. Wait for thirty minutes so that the unit and all its components are cool. Next, use your multimeter to test the flame sensor for a current in the range of 2-6 micro-amps. If the reading is not within this range, the sensor is most likely defective and will need to be replaced.

To clean the flame sensor, remove the furnace access panel and locate the flame sensor (it looks like a thin metal rod extending near the burners). Next, take a piece of emery cloth or light-grit sandpaper and rub the metal rod to remove soot and other residue from it. Be careful not to sand the sensor too much as it can be damaged or scratched.