Gas appliance or heater pilot light diagnosis and repair.

This article describes the reasons that the pilot light won't stay lit on some gas appliances and offers repair suggestions that range from quick and trivial like positioning the pilot light sensor properly to a bit more work like replacing the thermocouple sensor itself.

This article series explains what a thermocouple (or thermopile) is, how these similar devices work to support a pilot light on a gas burner, where they are installed, and what goes wrong with thermocouples.

Gas Pilot Light Troubleshooting & Repair

Article Contents

• CHECK FIRST - try re-lighting the pilot & watch for any of these three cases
• THERMOCOUPLE DIAGOSTIC CHECKLIST- PILOT WON'T STAY LIT - pilot light used to work but now keeps going out or won't stay lit
• THERMOCOUPLE INSTALLATION SNAFUS - pilot light has never worked well
• THERMOCOUPLE FAILURE RESEARCH

Watch out: here are some typical manufacturer warnings involving lighting pilot lights on gas heating equipment, both propane and natural gas:

If you are not qualified, do not light pilot lights, perform service, or make adjustments to appliances on the Propane Gas system. If you are qualified, consciously think about the odor of Propane Gas prior to and while lighting pilot lights or performing service or making adjustments.

Sometimes a basement or a closed-up house has a musty smell that can cover up the Propane Gas odor. Do not try to light pilot lights, perform service, or make adjustments in an area where the conditions are such that you may not detect the odor if there has been a leak of Propane Gas. - source: EMPIRE INFRARED HEATER with THERMOSTAT [PDF] Models SR10TW, SR18TW, SR30TW

Troubleshoot Thermocouple on Gas Fired Heating Equipment

If the Pilot Light Keeps Going Out, First confirm that the problem is the thermocouple

Start by re-lighting the gas pilot light.

Typically the gas burner control is pushed-in or held in a spring-loaded position to force gas through the pilot light assembly to permit manually lighting the pilot flame. Instructions for your gas appliance control are usually right on the control.

If you don't know how to light the pilot light, it's easy and safe: details are

at GAS BURNER PILOT LIGHT PROCEDURE

Here is a typical example using the gas water heater control valve shown just above.

1. Turn gas supply valve to the appliance on. Watch out: if you smell a gas leak, turn the gas back off immediately and call for repair. Else continue.
   
2. Turn the gas control knob so that the PILOT position is opposite the index pointer.
3. Press the large red button atop the control down and hold it: this forces gas to flow to the pilot light.
4. Light the pilot light with a gas lighter or match - you may have to hold the gas "on" for a minute for gas to reach the pilot light assembly, and you will need to continue to hold the gas supply "ON" until the thermocouple has heated up - as much as an additional 15-30 seconds.
   
5. Release the gas control button.
6. Does the pilot remain lit or does it go out?

Case #1: If you are able to light the pilot on the gas fired appliance but when you release the gas control from it's "LIGHT" position the pilot immediately goes out, if there was a good solid flame that was clearly touching and heating the thermocouple,

that is, the thermocouple was properly positioned in the pilot flame, and if you are sure that you held the control in the LIGHT position long enough for the thermocouple to heat up normally (30 seconds is plenty), then most-likely the thermocouple is defective and simply should be replaced.

In that case,

see THERMOCOUPLE INSTALLATION or REPLACEMENT

But if you want to be thorough and meticulous, read through the Thermocouple Problem Checklist below on this page first. You'll feel as silly as I do [DF] if you go buy a new thermocouple and then at the start of the replacement procedure you find that the existing thermocouple simply wasn't tightly-connected at the gas valve!

Case #2: If you are able to light the gas pilot and it stays lit for minutes or longer but eventually goes out, the cause is probably one of those listed just below.

Before reading through our complete list of gas pilot light thermocouple troubles, check these most-common issues:

1. The thermocouple sensor tip is not properly located and secured in position in the gas flame
2. The thermocouple connecting tubing has been mechanically damaged, crimped, cut or nicked or is sooty from a dirty or improper flame.
3. The gas flow to the pilot light is obstructed:
   

   Watch out: Before trying a new thermocouple check for debris in the pilot light orifice or tubing.
   
   In gas fired equipment that remains shut down for long intervals we sometimes find spiders or insects have nested in the equipment, even mud-dauber wasps, blocking proper gas flow or gas appliance venting.

   Photo at left: red arrows indicate the thermocouple tubing and assembly; blue arrows indicate the pilot light tubing & assembly on a horribly rusted Holland brand gas furnace.

   We have run into this problem and also read other accounts of it concerning the Gaffers and Sattler Model S 80 FDF gas fired furnace and similar gas fired heating equipment but this debris clog problem is widespread and may show up on just about any pilot-lit gas fired appliance, even gas log fireplaces and portable heaters.
   

   Check for debris blockage right at the pilot light, inside the gas tube between the pilot light and the gas control valve, and using a fine wire (don't scratch or enlarge the orifice) try cleaning the orifice at the pilot light itself.

   Other symptoms of bad thermocouple or dirty thermistor on gas fired equipment:
   
   Short cycling or unexplained on-off cycling of heating equipment has also been traced (by one reader) to a dirty or failing thermocouple (possibly the reader meant a dirty thermistor).
   
   See THERMISTORS
   
   This condition was reported in the FAQs by a reader commenting
   
   at HEAT ANTICIPATOR ADJUSTMENT
   

If you don't immediately see case 1 or 2 or 3 above, continue with our complete diagnostic list of gas pilot troubles given below on this page.

Case #3: If you are able to light the gas pilot and it stays lit but the main gas valve never opens

and the main gas burner won't ignite, it's possible that the thermocouple tubing has not been properly-connected at the gas valve end.

In this case you can light the pilot flame and it will stay lit, but the gas valve will never open.

Thermocouple Problem Checklist - What causes of loss of the gas pilot flame?

Reader Question: Question: why does the pilot flame keep going out on my hot air furnace?

What would cause standing pilot flame to keep going out . thermocouple was replaced on my air furnace - Bruce 11/17/12

If your gas appliance pilot light worked previously but now keeps going out, check the causes listed just below.

If your gas appliance pilot light has NEVER worked properly and has had problems with "going out" since the heater or thermocouple was first installed, check our second diagnostic list

at THERMOCOUPLE INSTALLATION SNAFUS.

In residential appliances such as gas boilers, furnaces, ovens and water heaters, the causes of thermocouple failure are listed below, roughly in order of probability, with the most-probable listed first.

Gas Pilot Light Won't Stay Lit

• Thermocouple sensor tip out of position
  
  not properly mounted, secured in the flame path. The sensor tip should be surrounded by the pilot flame.
• Thermocouple's indicating end loose
  
  not fully secured into the gas valve. Not fully seated.
• Thermocouple indicating end damaged
  
  at the gas valve control, if the sensor connector has been cross-threaded, it may seem to be fully tightened but in fact is not screwed fully in place.
  
  Watch out: If the thermocouple end at the gas valve has been cross-threaded, the threads may have been damaged so that you may never obtain a proper connection to the gas valve: both thermocouple and the gas valve itself may need to be replaced.
• A bad replacement thermocouple
  
  or thermocouple tubing that was bent, kinked, damaged during installation
• Sudden drafts blow out pilot
  
  We often find this problem in southern climates where heating equipment may be installed outdoors or in a shelter exposed to wind.
  
  If there is no chimney cap, other site properties (slope, obstructions) could cause the wind to blow down the chimney and extinguish the pilot light.
• Low gas pressure
  
  results in a weak pilot flame, the flame will be small and more easily extinguished. Low gas pressure or flow rate at a gas heating appliance might be caused by
  
  -- improper adjustment of the pilot light: some pilot lights such as on gas stoves have an adjustment screw that makes the pilot flame larger or smaller. Too small = easily blown-o0ut
  
  -- improper gas regulator adjustment? Unlikely but possible. When a gas burner is "on" its gas use is enormously greater than when just the pilot alone is on. If the burner flame is adequate then there is almost certainly plenty of gas supply for the pilot alone
  
  -- running out of LP gas? If your gas supply is from a local propane tank, and if the tank is very close to empty it's possible for the LP gas pressure to fall below the normal rate delivered by the LP gas regulator. Check the gauge on your LP tank. If it's at or close to empty that could be the trouble.
• Inadequate combustion air
  
  Watch out: this produces dangerous carbon monoxide, very dangerous, can be fatal
• Dirt on the equipment burner or on the pilot light assembly
  
  thermocouple, connections or at control wiring can cause intermittent failures of the pilot and gas valve.
• Dirt or debris or insects
  
  (spiders) clogging the pilot orifice or pilot light gas tube
• Thermocouple Sooting
  
  from improper gas flame adjustment, causing improper temperature sensing
  
  Watch out: a sooty gas flame is a sign of dangerous and improper burner operation, risking production of fatal carbon monoxide.
• Thermocouple Oxidation or Corrosion
  
  may occur in some locations where there are unusual moisture or corrosive gas sources; Oxidation inside or at joints, occurring if the thermocouple sensor or tubing connections are bent, cracked, damaged, leaky.
• Failed gas control valve
  
  less likely problem, the valve fails to open to deliver gas to the burner even though the thermocouple is properly connected and working.
• Water heater (or boiler) leak
  
  Particularly with a water heater, and less often a boiler, a leak in the water heater (or boiler) that happens to drip right onto the pilot light can put out the pilot flame.
• Thermocouple Burnout
  
  due to exposure to higher temperatures than the particular thermocouple was designed to handle. Possibly the wrong thermocoule type was installed.
• Metal fatigue -
  
  on older installations, more-likely if the thermocouple is exposed to vibration or movement or abrasion or where the parts are exposed to extreme temperature changes

Thermocouple Installation SNAFUS

• Damage during installation,
  
  such as sharp bends, nicks, gouges in the thermocouple tubing or failure to properly route the thermocouple tubing so as to avoid damage
  
  Note: some equipment or appliances include a protective grommet intended to protect the thermocouple tubing from damage where it passes through openings in the equipment. Be sure such grommets, if provided, are in place and intact.
  
  Damage due to cross-threading or failing to provide a thermocouple with the proper thread direction.
• Improper installation:
  
  wrong placement of the sensor, gas valve connection under-tightened or over-tightened (stripped)
• Damaged wiring
  
  or wire connections at the gas control (not actually a thermocouple issue)
  

• Something else we haven't thought of - please use the page top or bottom CONTACT link to add to this list.

Research on Thermocouple Failure Causes

• Anderson, R. L., and R. L. Ludwig. Failure of sheathed thermocouples due to thermal cycling. No. CONF-820306-5. Oak Ridge National Lab., TN (USA); Oak Ridge Y-12 Plant, TN (USA), 1982.
• Gates, K., and S. Benhaddad. "Failure analysis of a high-temperature thermocouple." Journal of Failure Analysis and Prevention 5, no. 4 (2005): 14-19.
• Gaffers and Sattler cited above in this article, was an appliance brand (kitchen ranges, heating equipment, air conditioning) owned by Maytag and actually preceded "Maytag" as a company name.
  
  The Gaffers Sattler and Maytag Washing Machine Company was founded in 1893 by businessman Frederick Maytag. G&S cooking ranges were identified as a subsidiary of MagicChef in 1969. Magic Chef found its way back to Maytag in 1986.

  If you are looking for parts for Gaffers and Sattler equipment check with Maytag. Separately a Gaffers & Sattler appliance company still does business in Murray UT.

• Mandal, Shyamapada, B. Santhi, S. Sridhar, K. Vinolia, and P. Swaminathan. "Minor fault detection of thermocouple sensor in nuclear power plants using time series analysis." Annals of Nuclear Energy 134 (2019): 383-389.
• Zhong, X. Y., X. Q. Wu, E. H. Han, and X. B. Song. "Corrosion failure analysis of pure nickel thermocouple sheath used in BaTiO3 hydrothermal synthesis reactor." Engineering Failure Analysis 17, no. 6 (2010): 1404-1412.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2022-07-29 by InspectApedia-911 (mod) - Can the copper tube touch the iron case of the oven?

@Diego,

That's an interesting question.

We're in the realm of OPINION here, as I haven't found authoritative research data on abrasion or oxidation damage to the thermocouple tubing where it passes through appliance openings.

Post some sharp photos (one per comment) of your old thermocouple so that we can see what you see and perhaps a photo as well of how the tubing passes through parts of your gas oven.

The typical thermocouple installation instructions given on this page at

https://inspectapedia.com/heat/Thermocouple_Replacement.php#ThermocoupleInstall

do discuss routing.

The tubing needs to be protected from mechanical damage, and extra length safely coiled and placed out of harm's way.

Needless to say experts have researched thermocouple failure. I'll add some of that to the article above.

thanks for asking

On 2022-07-29 by Diego

Great site !

I have replaced the thermocuple succesfully on my kitchen gas oven. It works right.... But when I inspected the old one I have noticed that its copper tube was degradated just wher it comes out of the oven, a 1,5 hole for a 4 mm diameter tube....

Can the copper tube touch the iron case of the oven or that will produce another thermocuple side effect that produce the oxydation in that contact point ?

Or is the water produced in the gas combustion ? Or perhaps a minimmal almost invisible spark from the thermocuple current between the two metals ?

Hoping being clear with my doubt... Thanks !

On 2020-11-29 - by (mod) - flickering pilot light - clean the pilot assembly

I'd clean all of the components, tighten the connection at the gas valve, and if the flickering continues I'd replace the thermopile or thermocouple.

On 2020-11-29 by Michelle

I am troubleshooting an older Williams wall heater. The thermopile “flickers” when it connects with the the lit pilot light.

The pilot light stays lit and steady with the “flickering” thermopile. Flame color is good. The heater works well. The continuous flicker noise is disturbing. Ideas?

Karen

Take a look (and post a photo) of where those two black wires lead. I am GUESSING that they lead to a couple of spill switches that might be located on the draft hood of your heater or right near the burner opening.

Those devices sense dangerous spillage of flue gases, are usually heat-activated, and some models require that the switch be manually-reset by pressing a reset button on the switch after it has cooled.

See details at FLUE GAS SPILL SWITCH TRIPPING & RESET

Let me know if that helps you out.

[Click to enlarge any image]

On 2020-10-13 - by (mod) - Pilot Light Control for old Luxaire furnace using a White-Rodgers 21D18-3 gas system.

Jerry

I don't know what's wrong for sure, Jerry,

a bad thermocouple, a bad thermocouple connection to a gas valve, even a bad igniter or igniter wire or igniter module can all cause the symptom that you describe. Sometimes when a component is failing it is more-vulnerable to moisture or to low temperature, and it will behave differently once warmed-up.

That can explain why your furnace will re-start OK on subsequent on-cycles but won't start up on its own when it's been off for a longer interval.

I'd consider replacing the control; you might want to take a look at the White Rodgers replacement option for the original 21D18-3 gas ignition system described in this

WHITE RODGERS RETROFIT for WR 21D18-3 GAS CONTROL [PDF]

Near the end of those instructions is a helpful gas burner ignition troubleshooting flowchart that I recommend.


I will re-post this Q&A at GAS REGULATORS & APPLIANCE / HEATER CONTROLS inspectapedia.com/plumbing/Gas_Regulators.php

as that article may also be helpful to you

On 2020-10-13 by Jerry

Hello.

I have an old Luxaire furnace using a White-Rodgers 21D18-3 gas system. If the furnace sits idle for any length of time (say overnight) the furnace is unable to produce any spark whatsoever to ignite the pilot. However, once I light the pilot manually and the furnace runs through a cycle, the furnace can produce a consistent strong spark on subsequent cycles, as long as the furnace has not sat idle for too long between these cycles.

It reminds me of a car with a failing battery. You might require a charge (or a jump) in the morning to get it going, but once started you can run any number of errands, successfully restarting the car a number of times. But let it sit overnight, and in the morning another jump is needed.

This have vexed me for some time and I would be thankful for any guidance.

On 2020-03-27 by Karen

Thank you for your answer of 5/2/19 to my on going problem.

This thermocouple has a little side thing on the connector that has two black wires. Now I cannot get the pilot to even try to light. I seem to remember reading something about them being for safety or something.

I had to leave the door open to the heater because the connector sticks out too far and when moved the pilot goes out.

A little giggle and it would work for awhile. Something is loose inside, I think.

Any advice. I could replace the thermocouple again, but now I cannot finknob colors may be red not black) d one with that extra wire thing. Do I have to have that? I have added a picture [above] for you. Thank you.

On 2020-02-24 - by (mod) - thermocouple keeps failing too frequently - was the tube connected properly at the gas valve?

Tony

It would be unusual to get a batch of bad thermocouples such that you have to keep replacing them because of an inherent defect in the thermocouple itself. Not unless they were all made by the same company in the same batch.

When I have run into this recurrent thermocouple problem before it has usually been traced to a problem at one of the two ends of the thermocouple.

One end, at the pilot, needs to be located exactly properly, mounted and placed in the flame of the pilot light.

The other problem at the other end of the thermocouple is more subtle. You can think that you have fully screwed in the connector that pushes the end of the thermocouple into the bottom of the gas valve but it may not actually be fully seated. That could be mistaken for a thermocouple failure, as poor contact can get intermittent or poor thermocouple performance.

It's also possible that the gas control valve itself is damaged or its thermocouple connector threads cross threaded or damaged so that you can't get a good thermocouple connection. That is a less common problem in my experience.

Lastly, it's possible that the defect is in a gas flow or clog problem in the pilot light itself.

On 2020-02-24 0 by Tony Risolo

I have had to replace my thermocoupler 3 times in 4 years the last one only lasted 3 months can you recommend a brand name and why is this continuing to happen My gas boiler is 35 years old and in good condition The boiler is a Hydrotherm

Question: furnace pilot goes out

(Nov 3, 2014) Todd said:

lennox g12 furnace pilot light keeps going out. sometimes it will stay lit for weeks sometimes not even a day . Ive cleaned Ive checked for fresh air. My only thought is when it calls for gas, the gas shoots out to fast and blows pilot out.

Reply:

Todd

Watch out: what you describe sounds dangerous - unsafe. I would shut down the system and have ot repaire. Sounds like a gas regulator problem.

(Nov 5, 2014) Todd said:

I am fully aware of the hazard. And if I could simply afford to have it repaired , I dont think i would be on here trying to get help. sorry if I seem rude but not a flipping idiot.

Reply:

Sorry, Todd. Please appreciate the iffiness of giving repair advice for remote systems involving life safety and where only very limited information about site conditions is known. No one thinks you're an idiot. A service tech would probably check gas pressures, the regulator, piping for blockages, etc.

The simplest repair that's worth trying first is to just replace the thermocouple that senses the flame. It's a standard part - just buy the proper length and check the mount fittings that hold the sensor element in the proper position.

...

Continue reading at THERMOCOUPLE REPAIR / REPLACEMENT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see THERMOCOUPLE TROUBLESHOOTING FAQs - questions & answers posted originally at this article

Or see these

Recommended Articles

• FLUE GAS SPILL SWITCH TRIPPING & RESET
• GAS BURNER FLAME & NOISE DEFECTS
• GAS BURNER SOOT CAUSE & CURE
• GAS BURNER PILOT LIGHT PROCEDURE
• GAS FIRED HEATING BOILER PROBLEMS
• GAS FIRED WATER HEATERS
• GAS FIREPLACES & GAS LOGS
• GAS IGNITER DIAGNOSIS & REPAIR
• GAS REGULATORS & APPLIANCE / HEATER CONTROLS
• GAS WATER HEATER CONTROL VALVES
• THERMISTORS
• THERMOCOUPLE REPAIR / REPLACEMENT

Suggested citation for this web page

GAS PILOT WON'T STAY LIT at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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Citations & References

In addition to any citations in the article above, a full list is available on request.

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