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How to Test your Thermopile

How to Test Your Thermopile sensor

Time to Complete:  5  minutes

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So you’ve bypassed the wall switch, the pilot flame looks good, and the main burner still wont come on.  Chances are, your thermopile sensor is faulty.  Just like your thermocouple, your thermopile will generate voltage when heated up by the pilot flame.   We can test the voltage that the thermopile is giving off with our Digital Multimeter.

You will want to test your thermopile leads.  They are connected to the gas control valve so first we should locate the valve.

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The main control valve is typically located underneath the lower louver or grill of the fireplace:

 

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Your Thermopile sensor will have a wire that runs from the pilot assembly, down to the main control valve.  It consists of two wires that usually have a metal or tan fabric sheath that protects the wires.

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The end of the thermopile wire branches off into two leads.  These are usually red & white wires:

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Another way to locate your thermopile leads is to look at your valve.  Your thermopile should be hooked up to the TP terminals on the valve:

 

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With the leads still attached to your valve, you can test the voltage on these (again, there is less than 1 volt of electricity so there’s no danger of electrocuting yourself).  You’ll want to put your Gardner Bender Digital Multimeter on the milivolt setting:

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Then take your multimeter leads and place them on your sensor leads.  It does not matter which one goes on which:

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With your pilot light running and your wall switch or remote in the “off” position, you should be getting at least 300-350 millivolts.  If it is less than that you may want to think about replacing it.  Here we can see we are getting  almost 850:

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Next, turn your wall switch or remote on, just like you would if you were turning on your fireplace.   You should see the voltage drop roughly in half:

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What does this mean?

You should see the voltage drop roughly IN HALF:

1.)If it zero’s out, or goes very low, you know you have a short in your thermopile and you should replace it. 

2.) If it doesn’t move at all, you may want to double check your wall switch or the wiring to it.

3.) If the voltage only drops a little bit, this may be an indication that the valve is bad.  There will be more on this in the next section.

 

A Note on “Snap discs”

Although uncommon, some fireplaces (mostly natural vented units) will have a heat sensitive “snap disc” that is wired in line with the thermopile.  The point of the snap disc is to break the thermopile connection if it gets hot enough.  Snap discs are used as safety mechanisms and will automatically cut the flame out if it is sensing the fireplace unit is not venting properly or if the fireplace unit is overheating.

An example of a fireplace snap disc:

Issues arise when the snap disc malfunctions.  The main burner will not turn on if the snap disc is tripped.

To determine if you have a “snap disc”,  follow your thermopile wires from the valve, back to the pilot assembly.  If either wire branches off and is not directly connected from the thermopile to the valve, it more than likely goes to a snap disc.


50 Comments
  1. Fixed the problem. Push In type wire had disconnected from the switch. Reconnected and all is working again.
    Thanks for this information.

  2. Great article, thank you.
    Pictures are a great help!
    Was able to test and confirm faulty thermopile in just minutes.
    Hardest part was to find one in Canada.

  3. Hi, my instaflame fireplace isn’t working…

    The pilot stays lit, but it won’t fire up when the switch is turned on. I cleaned the thermopile, thermocouple and the connectors from the thermopile to TH/TP, but it still didn’t work. I then got my multimeter and checked voltage from the thermopile as you outlined here, it went up to about 700mv, but when I turn the switch on (or jump the TP/TH) it dropped to about 10mv and the burner doesn’t fire. No matter what I tried, with the switch is on (or jumpered) the voltage drops to 5-10mv and the fireplace doesn’t turn on.

    Do you think this could be a problem with the thermopile? Could it possibly be anything else? I just want to make sure this sounds correct before I buy a new one.

    Thanks!

    • You are correct. There is a short in your thermopile. Change the thermopile and you should be all set.

  4. I replaced the thermocouple and the igniter would not spark. So I lit the pilot manually & the Igniter would then spark?? The fireplace functions properly like this. But, in warm weather I turn the pilot off and then have to remove the glass windows and relight it.
    Why does the igniter not spark unless the pilot is on?? Any idea?? Thank you, Leif

    • Your ignitor and pilot hood sound like it needs some cleaning. Take some emery cloth and clean the end of the electrode and the pilot hood (take care as to not crack the porcelain around the electrode). The ignitor needs a good clean surface to spark against. If you are still having issues try bending the end of the electrode closer to the hood (again, be gentle as to not crack the porcelain around the electrode)

  5. Hi my pilot light is on but fireplace won’t light I by passed the switch ,still nothing . When I check the thermopile with the meter there was no change the reading was around 578 . I checked all the wiring to the switch and all looks fine , any suggestions ? Sometimes the switch will turn it on and after awhile the fireplace will go off and on

    • If your voltage doesn’t drop, its most likely something in your wall switch or wiring.

      Try bypassing the switch at the main control valve on the fireplace (instead of on the wall).

      Take a piece of wire and connect one side to the upper terminal (TH) and the other end to the lower terminal (TP/TH) (these are usually the outer two terminals). This way we are bypassing the wall switch as well as any of the wiring leading up to the valve.

      You should see some sort of voltage drop when you do this. If it barely drops your main control valve is probably bad. If it drops to nothing your thermopile probably has a short in it.

  6. We have a problem with the pilot light it will light for a minute or two and the flame comes on and then it all shuts off.

    • How does the pilot flame look? Try visiting the . Many times if the pilot assembly has been deteriorated, once the main burner ignites and the fireplace establishes draft, the pilot flame will pull away from its sensors and the fireplace will shut down.

      • This is what’s happening with my fireplace. Once the burner comes on and a draft is established, I can see the pilot light getting pulled away from the thermopile, then everything shuts off. What needs to be replaced and what is the proceedure?

        • There are 3 things you can do. First, the pilot assembly can be replaced. A weak or “candle-like” pilot flame will be easily pulled away from the thermopile. If your pilot flame looks okay, you can usually order a restrictor disc from the manufacturer that goes in the fireplace that slows down the draft. Or you can make a pilot shield (you can sometimes order these from the manufacturer as well) to go around the pilot and protect it from getting drawn away from the thermopile.

  7. I have an older gas fireplace (10yr)its a STARDANCE DIRECT VENT STOVE. After moving into this home Ihad to restart propane service, the pilot wouldnt lite up so I used a BBQ LIGHTER, But upon turning on the switch(back of stove) sometimes the flame ignites and sometimes NOT, after burning awile it goes out, so I TAPthe side of the STOVE and the flame ignites(for alittle while) then I TAP AGAINand flames up!! I repeat this every 20/30 minutes!! what is the PROBLEM??PLEASE SOMEONE WERE REALLY COLD ! thanx two elderly ladies . email BestUcanB1@gmail.com

    • Have you tried replacing the switch on the stove? The little toggle switches can go bad just as easily as a standard wall switch.

  8. Thank you so much for this article site/article! I’ve tested everything except the Main Control Valve – as the link above continues to take me back to this page. Switch is fine, pilot is fine, tested thermopile and it didn’t move between on and off switch position. Reading was about 500. So I bypassed the switch using wire, and the voltage dropped to 10mv. Now here’s the thing. If I flip my switch to the “on” position, and then tap the side of the Main Control Valve (close to where the thermopile connects) it will ignite. The hiss of the valve opening begins and violá, it ignites. So I was wondering could it still be the Main Control Valve, since I didn’t find a way to test that? Again, thanks so much!

    • It sounds like the valve could be bad. I will be finishing the “how to test your main control valve” post shortly.

  9. I have an old Heat n Glo 6000 DVT. The remote control is not working and I have replaced the batteries. In checking the millvolt output of the thermophile I am seeing 325m.v., which is what the owners manual says I should read as sufficient voltage. What voltage is required for the remote to operate. It will turn the valve off, but not turn it on.

    • What happens to the millivolt output after you try to turn the fireplace on with the remote?

      • Thanks for the response. I just tested the thermopile and was getting 380 M.V. When I tested the remote and it opened the valve and the burner lit. The reading was 150 M.V.
        Could the problem be in the thermopile? When tested yesterday I was getting around 325 M.V. and now getting 380 M.V., just enough to activate the remote? Thanks again for your response, this site is great!!

        • Do you think the 380 mv made the difference in the remote working??

          • Its possible. 325 millivolts is just on the border of too low. Sometimes there are batteries in the reciever as well as the remote. Make sure you change both.

          • Actually the receiver is 110 AC. Thank you for your input, much appreciated.

  10. Thanks for the awesome pics ,It makes it so much easier ,I went to Ace Hardware and bought a thermopile (generator) Ace brand for $35 and worked perfect !!!

  11. By passed the switch and nothing. Thermopile reads 590 and no change when switch is turned on. Connected the TP to TP/TH with a piece of wire and the fireplace ignites. What is my issue? Any help is very much appreciated.

    • Figured it out. One of the wall switch wires went to the valve terminals, the other to a lower door saftey mechanism that prevents the burner from working when the lower door is open. That mechanism was bad. Bypassed the door mechanism and placed both wall switch wires directly to the valve terminals. Works fine now.

  12. Pilot light lights and stays on. Stove lights and stays on for a couple of minutes. Both the pilot and stove go out. If I redo the whole scenario same result??

    • Does the flame look weird before it goes out? If the flame turns blue and appears to lift off the burner and burn in the air, this would indicate a venting issue. Open the bottom of the glass assembly to allow the fireplace to draw room air to confirm this.

  13. The wall switch works, the pilot works, the thermopile produces 699mV when the switch is off and 335mV when the switch is on.
    The Valve or something gos klick but noting appen no flame

    Read more: http://www.mygasfireplacerepair.com/2012/11/15/how-to-test-your-thermopile/

    • Make sure your burner tube is not plugged. This involves disconnecting the main burner tube, running a wire through it to clear the obstruction, and reconnecting it to the valve. (if you feel uncomfortable you can always call a qualified service technician).

      Try turning on the fireplace (just for a second) with the burner tube disconnected to see if any gas comes out to determine if you have an obstruction or if the problem is in the valve itself.

  14. I read everything, and your posts are very helpful. I opened the control box and changed out the double A batteries. I changed the triple A batteries to the remote. But the control box light does not turn on. I have it on the remote setting but the physical remote does not communicate with the box to turn off the flame/fireplace. I have a heat and glo fireplace model 6000GDVFL. The only way I was able to turn off the fireplace flame was by turning the pilot dial from on to off. I then flipped the wall switch once the flame went out and it sounds like a fan came on. When standing in front of the fireplace with the switvh on the hotair blows out the top of the vent. May you helpnme to sync the remote to the control so I dont havebto manually turn on and off the fireplace with the pilot knob? Also thr pilot is a nice blue flame. Thanks!

    • You may have to “re-sync” the remote with the control box. There should be a small “learn” button you can press with a paperclip, then you can press any button on the remote to learn it. Also make sure your remote is not in a thermostat mode.

      It also sounds like the switch on the wall was for the blower (this is usually separate).

  15. I have a gas fire place it vents in the house. The pilot light works fine. I turn the switch on and the gas fire place works great after 10 minutes it automatically goes out and turns off pilot light and burner Any ideas ? Took fire place apart to clean put it back together and once again after 10 – 15 minutes pop it turns off again pilot light and burner
    Thanks for any help

    • It sounds like you have a vent free fireplace with an Oxygen Depletion Sensor or ODS. Most likely the ODS is malfunctioning and should be replaced.

  16. Great info.. thank you for taking the time to write it.
    I am confused with what I am finding..
    I can get the pilot on without a problem. When i turn the wall switch on nothing happens (most times).
    If I jump the two leads coming from the wall switch I get the same result. If I take a reading shown on this page the voltage is very low 135. if i turn on the switch i do not see a change. if I jump the two wires without switch .. no change.. if i use the rocker switch there is a change but minor. If I take a wire and jump the sensor leads it trips and shuts down the pilot.
    In the past when it would not go on.. if you hit the burner pipe it would light.. not it does not even do that. Suggestions?

    -Ken

    • If you’re not getting over 300mv you will need to replace your thermopile or pilot assembly (based on how the pilot flame looks).

  17. I am getting 540 when the pilot is lit, but when I go to turn the switch to on, the pilot goes out and obviously I guess, as the thermopile cools the voltage starts to drop. Any ideas? It worked fine a motnh ago.

    • By switch do you mean wall switch? or valve knob? If the wall switch is connected to the wrong terminals it will short out the thermopile when turned on.

      If the pilot goes off when turning the valve knob to “on”, your valve may be bad. Try turning the knob very very slowly to on. Many times this will keep the pilot running.

      • OK…I gave it one last try by re-hooking the remote receiver back up and while doing that making sure the thermopile connections were clean. Not sure why, but the fireplace turned right on when I moved the remote receiver switch to “ON”! So far, your column has fixed two fireplaces in my neighborhood this week alone. Thanks!

  18. Hi, I have 400mv across my TP leads with and without the switch on, same with switch wires tied. There is a snap disc, would that cause the same result as a bad switch wire connection? Thanks

    • Try jumping the valve directly. Use a wire between the TP/TH and TH terminal. If it ignites it may be in the wall switch wire itself or how they connect to the valve. If it does not, test your control valve.

  19. Thanks for the info – was able to use to diagnose and fix mine (turned out to be a faulty plate switch on the door.

  20. THANK YOU! THANK YOU! THANK YOU!
    This article walked me through step by step, and helped me to diagnose the problem that I never would have had a clue how to on my own. after going through it all, I replaced the thermopile and BINGO, I have a fire again!!!!
    Thanks again!

  21. This was awesome!!! You guys saved me a minimum of 200 bucks by sharing this easy to follow instructional.
    Thank you!!!
    Replaced the thermopile after ruling out the main valve and watching millivolts drop to zero when flipping the main switch on. Working like a charm.

  22. Just wanted to say THANK YOU! I used the information on this page to diagnose and succesfully repair my old gas furnace. My symptoms were a functioning pilot light but no main burner ignition. Tested the thermopile with the voltage tester and got around 330 millivolts while wires were unconnected to gas valve and with pilot light on. Then got around 218 with the wires connected. Finally when I turned on the thermostat to make it call for heat, I only got 6 millivolts…BINGO! Found a thermopile at my local hardware store for $35.00. Install took five minutes. YOU SAVED ME HUNDREDS! Also, plumbing and heating pros down the street refused to sell me the part without a service call, citing “insurance reasons.” A-holes. I hope this helps someone else.

  23. I know these posts are a year old but I found the site and it was excellent. My flame would not come on with all of your test showing good. I would hear sort of a click or some kind of sound when turning on the fireplace but no flame. I had to clean under the hood of the pilot with a thin wire although it did not feel clogged. To my delight after assembling the flame was there. Thank you!

  24. The wall switch works, the pilot works, the thermopile produces 600mV when the switch is off and 235mV when the switch is on. When the switch is on the fireplace partially lights. It is kind of weird. The pilot light stays on and a few of the holes where the blue flames come out are on but most of the holes are not producing blue flames. The blue flames aren’t all that tall, certainly not as tall as when it worked last season.

  25. You have blockage in your main burner tube. There is a certain spider that likes the smell of the natural gas additive (that sulfur gas smell) and will often build a nest in the main burner tube.

    To clear the blockage you usually have to remove the logs and burner to find the burner orifice (it is usually brass in color and looks like a bullet). Take a piece of wire and run it through the orifice and down the tube to clear out any blockage. If it is down far enough you may have to remove it from the valve itself.

  26. Awesome, thanks

  27. This comment saved me. I did have a weak thermopile but main burner was still not lighting. Cleared the line with a piece of wire and we were back in business. Thanks.

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