Thermocouple diagnostic FAQs for heating appliances:
Questions & answers on how to diagnose & repair or replace a bad thermocouple or thermopile.
This article series describes thermocouples, safety devices used on gas fired heating equipment.
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These questions & answers about diagnosing, fixing or replacing thermocouples on building heating equipment were posted originally at THERMOCOUPLES
On 2017-03-06 by Anonymous - fireplace thermocouple sensor diagnosis
Sounds as if your heater may be producing dangerous or even fatal carbon monoxide, shutting itself off.
Be sure you have working CO detectors properly located and tested.
Review the heater manual to see if it has a CO safety feature.
Also check combustion air adequacy.
On 2017-03-06 by Karl
Thanks - I pulled the glass today, cleaned the pilot assembly including the thermocouple and checked all connections. I cleaned and reinstalled the glass. The fireplace started and ran for roughly an hour, then the fire went out. The pilot remains lit, even after the fire extinguishes.
I can turn the pilot off at the switch. If I try to restart the fireplace the electronic ignitor starts the pilot light, but the main flame will not ignite.
If I wait until the unit cools, the fireplace will light up again. I guess I could replace the entire pilot assembly, and see if that fixes the problem. It's strange that the pilot light stays on, when the main flame goes out. It seems to me that if the thermocouple was bad, the main flame and pilot would extinguush.
On 2017-03-06 by (mod) fixing a Lennox gas log fireplace with an electronic ignition
I'm not sure what's wrong, Karl, perhaps a bad regulator or dirty gas orifices, but a common cause would be a bad thermocouple or flame sensor.
On 2017-03-05 by Karl
I have a Lennox gas log fireplace with an electronic ignition. The fireplace flame lights and burns for an hour, then the flame goes out.
The pilot light stays lit. It takes another hour before the fireplace will light again, and it stays lit for another hour. I've by passed the wall switch, so I know the switch and temperature control are not the problem. I'm baffled and need your help. Thanks in advance!!!
On 2017-02-08 by (mod) Sanding won't fix a failed thermopile,
Sanding won't fix a failed thermopile, but replacement would work IF 1. that was the problem and 2. the new unit was fully seated and its connector screwed in fully at the control-end of the device. I suspect there's still a problem either with the thermocouple connections or with its placement (not in the flame).
On 2017-02-04 by Bob Regan
I have a natural gas fireplace (with local wall switch and remote control receiver circuit). It just quick working suddenly, pilot self extinguished etc. I have ensured that the remote receiver has good batteries. Like a lot of other folks, the pilot will not stay lit.
I removed the thermopile and sanded lightly to make sure corrosion or carbon buildup was not preventing operation. I next replaced the thermopile with an identical one (at least, looked identical to me).
Well, the pi.ot still goes off as soon as I release the pilot switch. I removed the pilot valve cover to expose the coil and cannot see anything obviously wrong. The pilot valve is a Robertshaw brand with 0% turndown (2001 vintage) . Got any ideas/advice? My next move is to simply replace the pilot valve assembly....... then probably call a pro?
On 2017-01-18 by Anonymous
Pilot light lights but goes out when the burner attempts to kick on, any ideas?
On 2017-01-16 by (mod) thermocouple that keeps fireplace burning has snapped
If the thermocouple has broken off you will want to dash out to a plumbing or building supplier and buy a replacement one to get your heater going. Meanwhile just turn off the gas supply to that heater for safety.
A gas fireplace that is off would not cause other heating systems to stop.
On 2017-01-16 by Dean
Have no hot water or heating . Piolet light on ...but thermocouple that keeps fireplace burning has snapped
Would that course no hot water and heating rads. In airing cupboard pump wrks and hear it running ALL COPPER PIPES COLD
On 2016-12-28 by (mod) diagnose & fix millivolt/pilot flame controlled Robert Shaw gas valve
When you replaced the valve did you replace the thermocouple? If not I would do so.
Be sure you also have working and properly located carbon monoxide detectors as a malfunctioning gas heater can be fatal.
On 2016-12-28 by bafrank
I have a wall furnace with a millivolt/pilot flame controlled Robert Shaw gas valve, which has been working just fine as this heating season began this year. A few days ago it started not consistently responding to changes to the thermostat.
When I would manually raise the temp settings, something I do all the time, the read out on the thermosdtat would say "heat on" but the gas would not turn on. I could very lightly tap the control valve and the buner would ignight and the heater would run until the set room temp was reached.
Once the burner cut off it was hit or miss as to whether it would cut on again...without tapping the valve. I replaced the valve yesterday and the heater ran for several cycles. But this morning, when I over-rode the temp setting (battery operated programable thermostat) the burner failed to turn on.
A light tap to the valve precipitated ignition and the burner ran until the set temp was reached. As the room temp has dropped, the heater has faild again to ignite. The pilot/thermocouple control was replaced about 5 years ago. Ther previous unit lasted 20 years. I would like to know what to trouble shoot rather than replacing another part in a trial and error method.
On 2016-12-20 by Eddie
I am renting a house with gas water heater. It is an old water heater. The gas on/off switch is spring loaded according to the plumber that came out to see why I was getting no hot water after the air temperatures fell into the 40s and 50s here. He said the gas was turned off.
Now I am no dummy, if I had turned the gas off I would have simply turned it back on. He said the control valve is spring loaded. He had never seen or heard of it, but he said the on/off switch could have popped to the off position because of the spring.
Once he turned it on I suddenly had really hot water shortly after. In hindsight I never thought the water was 'hot'. Just OK warmth. Temps here over the summer were in the 90-100 range most days. Could it be possible the gas was never turned on and I was getting enough warmth with just the pilot light burning? Or is it possible the spring loaded valve did switch to off for some reason? Thanks
On 2016-12-19 by Cw
GE. gas water tank ignights to heat water but slower goes out after 3 to 5 seconds.
On 2016-12-02 2 by Jon
I have a older house gas furnace Carrier model #58GC080-1A The pilot stay's lit but the burner's don't come on. when you turn the heater on at the thermostat you hear the fan or blower come on but no heat,
Now at the furnace I connected a jumper wire between the red and white wire to bypass the thermostat same thing pilot light is lit blower come's on but burner's never light.
So it's not the thermostat could it be the thermocouple? can they go bad and the pilot stay lit and not let the burner light or is it the gas valve. and I been looking up part's and can't find anything for this thing.
On 2016-11-24 by (mod) gas fireplace has a thermopile and a "thermocouple"
thermocouples are pretty standard parts on residential gas heating equipment. I would remove the old one and take it to my Plumbing supplier for a match up.
On 2016-11-23 by Keith
My gas fireplace has a thermopile and a "thermocouple" which is a sensor at the pilot with a very thin single metal wire running to a round metal casing which in turn has 2 connectors for the connection to the gas valve. i get no v reading on the wire connectors so I am assuming the sensor needs to be replaced.
(Pilot flame will not stay on when I release the knob). Problem is I cannot find a replacement part, and the fireplace is about 25 years old. Can I get a thermocouple that ends with 2 wires and direct connect to the gas valve ? I have replaced the thermopile and the v reading for that seems to be good.
On 2016-11-16 by Spence
What causes a deposit to build up on the side of a thermocouple in a gas fireplace that eventually eats through the outer casing?
On 2016-08-19 by (mod) thermocouple connector to gas valve is reverse-threaded!
It's so that most thermocouples are reverse-threaded.
What's the brand you're using? Certainly thermocouples are a rather standard part used on many different gas controls, all of which are likely to use the same thread direction.
On 2016-08-18 by Konrad
well...my thermocuple keeps burning out
hasn't happen before, every one I put in now burns out after about one or two hot tank of water.
Turn the screw clockwise to loosen a right-hand-threaded connector such as this one <
no .anticlockwise to loosen a right handed nut, clockwise on a left handed like mine has.>
On 2016-03-08 by (mod) explanations for gas flame failure after installing a new thermocouple
Here are some possible explanations for gas flame failure after installing a new thermocouple
1. the end of the thermocouple that attaches to the gas valve is not properly seated or not tightened
2. the flame sensor is not adequately located in the body of the flame
3. the replacement thermocouple was defective or was damaged (kinked line) during installation
4. there is a different problem with the gas valve or gas pressure
On 2016-03-08 by John
Replaced Thermocouple but somehow pilot and or the flame still goes out
On 2016-01-31 by email@example.com
On my gas furnace, sometimes the igniter will spark for a longer period of time before a flame and sometimes it will eventually stop with no flame and sometimes it works fine. What could be the culprit?
On 2016-01-16 by (mod) continuous sparking then burns out the igniter
I suspect the installation is incorrect or a control board is bad. The continuous sparking then burns out the igniter. A grounded is shorted of wet igniter wire can also cause this problem.
On 2016-01-16 by Michael S.
Michael S. said:
I have a wood burning fireplace with fake logs which my wife likes to use every day. Years ago I had a contractor install this ignitor system so she did not have to use a match every day:
It worked fine for years, but then the ignitor/sensor went bad, so I bought this one
Lately I have had to replace this ignitor/sensor every few months, which clearly isn't right since they are supposed to last many many years.
What ends up happening every time is that the ignitor/sensor sparks, the pilot immediately lights, and the ignitor/sensor continues to spark and not open the gas line even though the tip is definitely in the pilot flame. The tip turns red from the heat of the flame, and it keeps sparking as if it cannot sense the flame of the pilot.
Even if I use a lighter to put an additional flame on the sensor, it still keeps sparking in the pilot flame.
I have tried cleaning the tip which never seems to fix it, so I end up just replacing it. This last one that I replaced did not even work for a week, so obviously I am using the wrong ignitor/sensor, or the wrong size orifice, right? What should I do so that this stays working for years, not days or months?
On 2016-01-16 by Clint
Thanks for this. My thermocoupler has two electric posts on it, what are these for?
Replacement at local appliance store didn't have those posts, so I did not buy it. Looks like this
On 2015-12-19 by W Weil-Mclain #119354325 the pilot is lit , water in phial, thermostat up but no ignition.
Weil-Mclain #119354325 the pilot is lit , water in phial, thermostat up but no ignition.
Pilot lit but noignition
Gas valve not opening.
On 2015-12-08 by firstname.lastname@example.org pilot stays on but oven won't fire up
pilot stays on but oven won't fire up. From what I've read here this doesn't sound like a thermocouple problem. What could it be??
On 2015-11-29 by (mod) In my natural gas fireplace, the pilot stays on fine, but the main flames cycle on and off at random.
IWatch out: don't know what brand and model fireplace you have - that'd help.
But generally it sounds as if you're describing a very dangerous situation, risking fatal aspyxiation or an explosion or fire. I would shut off the gas to the fireplace and leave it off until you've had a professional repair or replace its control or regulator or valve - or until another problem such as a gas leak is found.
On 2015-11-29 by email@example.com
In my natural gas fireplace, the pilot stays on fine, but the main flames cycle on and off at random. Are there two thermocouples for that? Othewise, what's your wisdom on that?
We have a 1907 rowhouse with a basement gas boiler fueling hot water radiators on the two floors above.
The boiler, a Utica Model 4GB-100, keeps blowing the thermocouple that senses the gas flame (one or two each heating season for the past six or seven years).
Now that we know what to expect, we don't mind replacing the dratted things periodically, but we wonder if this signals something we should be worried about. The long-term future of the heating system is not a big issue, since we likely will not own the house much longer. Our concern involves the living creatures inside it.
We don't mind paying you a reasonable fee for guidance/suggestions. But we're hoping you don't tell us to call in a local professional.
If the experiences of our neighbors are any indication, no one in our area wants to deal with these old "inefficient" systems, only to sell us a brand new one (preferably one that requires moving the boiler to the opposite end of the basement, thus requiring lots of pipes to be relocated).
We appreciate any advice or information you can supply. If you need more information, please don't hesitate to write or holler at us. - K.L. 10/23/2013 Oh, and we LOVE InspectAPedia.
A bit of detail might help me help you. Here are some diagnostic questions: Over what time have how many thermocouples failed? Give me a rough calendar.
Can you tell me the brand and part number you've installed?
Can you give me some sharp photos of the heating system - both in macro that I can see the heater's general environment and close-up of the control installation? I particularly want to see a photo of the position of the thermocouple sensor in the actual gas flame. Have you bought all of the replacement parts at the same place?
Did you see anything interesting on the old thermocouples like a kink, sharp bend, mechanical damage?
And no, there is no fee for my consulting as long as it doesn't become too consuming for me. Utica gas boiler information for your product series is at http://www.uticaboilers.com/products_gasboilers_mgb.asp
Take a look at the installation instructions at THERMOCOUPLES.
Also take a look at page 27 in this Utica MBG gas boiler installation manual http://www.ecrinternational.com/secure/upload/document/2783.pdf
at the pilot flame diagnostic drawing where you will see that placement of the thermocouple properly in the flame is important: let's also be sure that the problem is the thermocouple and not its position or the pilot flame.
(Jan 4, 2015) Anonymous said:
I have replaced the thermocouple in my gas wall heater but the pilot is not staying on. Is any adjustments at the valves?
I've had this trouble too: check for a thermocouple that is not properly installed: e.g.
(Oct 2, 2015) bill jackson said:
I just replaced the thermocouple on nat gas boiler but still can't keep pilot flame burning. I think the flame is strong and blue enough but flame goes out when the knob is released. what prevents the pilot from holding?
First check that the sensor is mounted well within the flame.
Then hold the ignite control to keep the flame going for at least 30 seconds.
If when you release the control from the "ignite" position if the flame goes out, then check for an improperly-seated thermocouple at the gas valve end.
(Oct 15, 2015) Frank Guttery said:
Pilot flame does not touch thermocouple all the time. When the flame does not touch the thermocouple the heater cuts off. Why does it do this?
This sounds backwards to me; I suspect a defective thermocouple.
2015/11/09 John said:
Just replaced pilot/thermocouple assembly on my reliance 606 water heater and pilot still blows out?
If you're sure the thermocouple is a good one (I sometimes try a second one) then check
- that the sensor is properly positioned in the flame
- that the flame is blue, healthy looking, and covers the sensor
- that the other end of the thermocouple is properly seated in the screw-in connector on the gas control
- that the thermocouple line is not nicked or kinked
I checked all of your suggestions to fix this thing. The pilot stays on then I turn the valve to on then it starts the heating cycle but when its done heating the pilot still goes out. Is there a separate thermopile generator?
Not usually on gas water heaters; I suspect the thermocouple is heating up and that is causing movement or change that's fouling things up. When the unit is still hot if you can't re-light the burner that may confirm.
I disconnected the thermocouple and pilot from the vavle and positioned the 2 so they're straight out of the grommet about 3/4 of an inch. Then recoiled the excess at the bottom of the valve and reconnected em. It seems to have worked. Thanx Daniel
Thanks for the follow-up john. I've occasionally had trouble with a thermocouple that seemed connected to the control but wasn't perfectly seated. Perhaps straightening out the tubing at the control end and re-connecting was what solved the trouble. I'll be sure to emphasize that for other readers.
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