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The pilot won't stay lit on my furnace, when I release the button it just goes out. This started occurring after I did some gas plumbing and had to relight the pilot. I suspected the thermocouple and replaced it, but this didn't fix the problem. I've used this furnace for over 15 years without replacing the thermocouple. Can someone recommend the next best course of action to keep my pilot lit?

Some things to note:

  • The new thermocouple is a cheap "universal" thermocouple that I purchased from Amazon and it doesn't seem to sit in place tight but I don't think it's moving.
  • There seems to be a secondary thermocouple-type wire running alongside the thermocouple and pilot gas line, though I ensured the thermocouple is not in contact with anything else.
  • While holding the pilot light button down I get a blue flame with a yellow tip that's over an inch in length. I've held the button down for over the recommended 60 seconds.
  • The thermocouple is below the pilot light, but the flame is definitely touching it.
  • The furnace is definitely over 30 years old and was likely installed in the 60's.

Control Module

Furnace Label

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  • What make and model is the furnace?
    – ThreePhaseEel
    Jan 7, 2018 at 5:18
  • It's a Lennox GS8D-105DM.
    – Hari S. Khalsa
    Jan 8, 2018 at 15:05
  • Seems this model was made between 1967 and 1969 nachi.org/forum/…
    – Hari S. Khalsa
    Jan 19, 2018 at 22:20
  • Some times with old lines particles can cause valve problems, Millivolt valves can fail for water in the line this is why we usually have a drip leg prior to the valve. I would see if you can hear the valve actuating if you can there may be some rust, moisture or even flakes of pipe dope that is preventing it from working. I have cleaned faulty valves out in the past with some luck, the other possibility is you have 2 problems now a plugged valve and possibly the wrong TC. If you can't clean it and get it to work a new valve can save the cost of a new furnace.
    – Ed Beal
    Feb 9, 2018 at 0:37
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    I'll note that sometimes the problem is that the pilot flame does not get the thermocouple hot enough, possibly because the pilot is not "aimed" at the thermocouple.
    – Hot Licks
    Apr 2, 2021 at 20:28

1 Answer 1

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If their is a good flame on the thermocouple then it is a bad gas valve. Sometimes if the gas valve is just barely bad a hotter thermocouple will work for a short time but finding a hotter thermocouple is just hit and miss. I have seen people try 4 thermocouples and the fifth one worked then they claimed the other four were bad. The furnace of course stopped working a few months later after the thermocouple mellowed a little.

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  • I added a picture of what it looks like. Is the gas valve inside the control module? Can I replace just the gas valve or do I need to replace the whole module? Do you think opening up the gas lines caused this? I really appreciate your response. Thanks.
    – Hari S. Khalsa
    Jan 8, 2018 at 15:20

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