How to test flame sensor

<a href=”Lennox 28M97 Angled Flame Sensor For Mpga Cmpe Cmpb 20467102 ” title=”flame sensor amazon”>How to test flame sensor

In the picture you will see a 1/8 inch round rod in the flame, that is the flame sensor. You need a multi meter that measures micro amps which you will connect in series with the flame sensor. The sensor is usually located on left side but on some furnaces it may be on the right side.

How the flame sensor works is by a small AC electrical charge going through the sensor and it uses the flame to carry the electrical power to ground. Burner surface to flame sensor is 4:1 ratio thus causing the AC current to change to DC current. If no flame present, dirty flame sensor, bad flame characteristics similar to flame lifting off or carbonizing there will be low or no micro amp draw and the control board will shut the flames off. After 3 to 5 tries the control board will lock the furnace out for 3 hours, which can be reset by cycling power or thermostat.

All furnaces have different micro amp readings some good at .8 while others need 6 you will need to find out the right micro amp reading for your furnace. The flame sensor has a coating on it to make it last a long time and will get worn off by over cleaning, so it is best to only clean rod when it is needed. If flame sensor requires cleaning more often then once a year it should be replaced, or check for other problems like vent stack temperature, gas pressure, or primary air need to be adjusted.

How to clean flame sensor

  • First find the flame sensor.
    Most furnaces you will find a white wire located on the left side connecting the flame sensor to the circuit board.
    Watch the furnace ignite you will see spark or orange glow in front of one burner and the flame sensor should be on opposite side, but I have seen them on the next burner from the igniter.
  • Shut power off to the furnace while you are working, you will find a light switch high on wall or in the ceiling.
  • Most furnaces have 1/4 inch screw that holds the sensor in place; any multi screwdriver without the bit will be 1/4 inch. Remove the screw(s) and disconnect wire
  • Once removed you can clean the sensor with a scotch brite pad, if you do not have one then just use a clean rag for now. Do not use sand paper or wire brush, as it will remove the protective coating and scratch the rod causing it to fail more often. Some people like to clean every year but I advice not to, It is best to clean only when needed since the protective coating will be removed with over cleaning
  • Once the rod is cleaned reconnect the wire and screws making sure the rod sits in the flame. Some times the burner and any grounding surface will get dirty and need to be cleaned as well.

Click here to see my post “first check thermostat”
Click here to see my Post on protecting your home “Alarm for home”

Thank you for visiting my blog I would appreciate any feedback I can get, Please leave a comment on whether my information has helped you. I am always interested in more topic to write so feel free to leave a comment asking about anything related to HVAC-R (Heating, Ventilation, air conditioning, refrigeration).

Thanks for reading my blog

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14 thoughts on “How to test flame sensor

  1. inommattVox 12/09/2012 at 6:42 am Reply

    You made some decent points there. I looked on the web for the concern and found most individuals will go along with along with your website.

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    • jetstrem 12/18/2012 at 8:48 am Reply

      Thanks for your comment, I try to make things easier for everyone to help them determine what is wrong and to make repairs. Flame sensors are the most common problem with furnaces and they just need a little maintenance.

  2. Mike 11/13/2013 at 7:40 pm Reply

    I have a Magic Chef G6A75D-11 that is exhibiting symptoms indicative of a faulty flame sensor (blowing intermittently, not responsive to thermostat temp changes). The furnace is VERY old though I do not know exactly how old, at least 20 years…at least. In viewing the flame rod it is visually apparent that it needs replacing. The flame rod appears to be crumbling and is jagged at the tip.

    I am having trouble finding a replacement. Are flame rod’s generally interchangeable among different models?

    The gas valve assembly is manufactured by Honeywell for this model.

    Any help is greatly appreciated.


  3. Vimax 11/16/2013 at 9:35 pm Reply

    Keep working ,great job!

  4. Tony Gallegos 11/25/2013 at 11:59 am Reply

    Thank you very much. You are spot on. I am an electrical engineer- so I understand the concept well. Some yahoos have posted that a flame sensor measures heat and thus puts out millivolts. No- that’s a thermal couple. A flame sensor measures current through the flame, so series current must be read. Very good- Take care.

  5. Interior noise measurements 12/07/2013 at 7:42 pm Reply

    Your style is very unique in comparison to other folks I have read stuff from.
    I appreciate you for posting when you’ve got the opportunity, Guess I will just bookmark this

  6. Scott 02/03/2014 at 2:51 am Reply

    Thank you for being so thorough. Over the past few years and certainly yesterday I have sanded off the protective coating. I’ll start the hunt for a replacement.

  7. Mike Kamenar 10/11/2014 at 7:28 pm Reply

    Hi, great post. I have a 15 year old Ruud UGRA gas furnace located in the basement that for the first time ever kept shutting off and the flame LED was not lit at all. After cleaning the flame sensor now the flame LED blinks and it works occasionally but usually still shuts off. It appears the blinking flame LED means the current is marginal. Would you suspect a bad/corroded ground? I actually ordered a new flame sensor yesterday but after reading your post it doesn’t sound like it should really go bad if the ceramic isn’t damaged. Could I use a temporary ground jumper wire from the burner to the control board to isolate it to a grounding issue? I tried wiggling all the ground wires to no avail. Thanks.

    • jetstrem 06/18/2015 at 1:06 pm Reply

      Flame sensors have a coating on them that will get worn off after it is cleaned or by age. I would try new flame sensor first. If still problems try temporary ground. Also check flame quality, sharp blue flame and not lifting off burner

  8. Tom Vardigan 11/19/2014 at 7:00 pm Reply

    Nice well written and well explained article. I was not aware of exactly how the flame sensor worked. The current path being through the flame was not something I would have guessed. The rest of my comment is exactly everything Tony said.

  9. Mark 05/27/2015 at 3:46 am Reply

    Thanks for sharing useful information

  10. Kcs 12/25/2016 at 10:40 am Reply

    Thanks. Very good information. Kcs

  11. nathaniel 01/04/2017 at 3:55 am Reply

    This article was spot on. The problem was exactly as outlined and so was the solution. I cleaned the flame sensor and it’s working as it should. Thanks a bunch!

  12. A@ndr3@ 04/30/2018 at 12:26 pm Reply

    I think I will have the havc tech read this because he, and other techs have cleaned the flame sensor at least 3 times this year, and it keeps getting dirty they tell me. And they don’t know why. I said change the flame sensor, they haven’t yet. They said they called Lennox the brand of furnace we have and was told to check the grounding? It goes into lockout once a month.

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