Pilot Light Troubleshooting

This section is a tutorial on how to keep your pilot light to stay lit once you initially light it.  If you cannot get your pilot to light, please refer to the How to Light Your Pilot Light Tutorial.

There are a few reasons why a pilot light will not stay lit. But before we explore them, we should first learn how a pilot light works.  For a more detailed description of how gas fireplaces work please visit our Fireplaces 101 page.  But before continuing with this tutorial, I highly recommend checking out the how a pilot light works link.


If your pilot light will not stay lit, it can only be 3 problems.

  1. Your pilot flame is bad
  2. Your thermocouple sensor is bad
  3. Your main control valve is bad

Just from experience, 99% of the time, the first two are the case.  It is very rare for a main control valve to go bad (although occasionally it does happen).  The good news is that each of these parts can be tested!

Personal Note: The fact that we can test components is very important.  Many times I would find that many “professional” fireplace service or heating and cooling companies would not know how to test these components.  I would often be the second or third company called to try to fix somebody’s gas fireplace after other companies were unsuccessful.  In fact, my training when I started servicing fireplaces consisted of a “replace parts until it works” strategy which some companies often employ.  This is part of the reason why I created the Find Certified Technicians page; so that if you choose to hire a professional you are more likely to get someone who knows what they are doing.

  1.  How to test of your pilot flame is bad

This is probably the easiest of the tests, and it doesn’t require any equipment.  You just need to visually inspect your pilot light.  It should be blue, without much yellow in it at all.

Time to Complete:  >5 minutes

Recommended Tools

  • Your Eyes


A healthy pilot flame should look like this:

It should be blue and intense and the flame should engulf the sensors surrounding it (see how the flame completely covers and wraps around the thermocouple sensor on the left and thermopile sensor on the right).  This is important because those sensors need to be a high enough temperature to keep the gas flowing to the pilot.  NOTE: some fireplaces will not have a thermocouple AND a thermopile.  Some fireplaces just contain a thermopile (the larger one on the right side of the pilot flame in the above picture).  The same idea applies to this setup as well.  It all starts with a healthy pilot flame.

Conversely, an unhealthy pilot flame can be easy to spot.  Here is an example of an unhealthy pilot flame.  It is generally very yellow and large like this:


This is not a good thing.  If you look at the picture, the pilot flame barely touches the thermopile sensor.  The thermopile sensor may be functioning correctly but the pilot flame is not intense enough to heat it up to the proper temperature.

In a nutshell, this is the easiest way to describe it:

A healthy pilot flame will be much like a blue  “blow torch” flame. It will shoot sideways and be directly on your thermocouple and thermopile sensors. An unhealthy yellow flame is more like a “candle”. It will shoot straight up in the air and may not engulf your sensors.

If the pilot flame is not healthy, it will not engulf the thermocouple or thermopile and the result: a pilot that wont stay lit once you stop pressing in the pilot knob.

If you determine that you have a bad pilot flame, you can either clean out your current one with a wire brush (cheap but not long term) or replace the pilot assembly (orifice and hood).  Cleaning it out would require taking the pilot assembly apart, taking the pilot orifice out, and cleaning everything with compressed air and the pilot hood with a wire brush.

Replacing the pilot assembly takes a little more time but will last much longer.  Find out your Make/Brand and model of your fireplace.  You can visit the What Kind of Fireplace do I Have page if you are having trouble.  You can find replacement pilot assemblies here:

2.) How to Test Your Thermocouple Sensor

If your pilot flame looks good, chances are you have a bad thermocouple sensor.

This sensor controls the pilot flame.  Again, please refer to the how a pilot light works section if you do not know which one is the case.

For this test we will need a device to test voltage.  As we learned in the how a pilot light works section, these sensors create a small amount of voltage.  By testing the voltage on your thermocouple, you can determine if it is malfunctioning.


Time to Complete:  15 minutes

Recommended Tools

Here are a few tools that I recommend.


For this tutorial, I am using the Gardner Bender Digital Multimeter .  You can purchase this through my affiliate link above for about $18.00,  otherwise I have used this one before which runs about $2.00.  The $2 one should do the trick but I have had it give me false readings before, so just keep that in mind.

Also, you basically just need a 7/16” wrench.  The jumper wires just make it easier for testing.

First, you want to locate your main control valve.  This is the same control valve that you use to light your pilot.

Gas fireplace main control valve

Next, you’ll want to locate the thermocouple sensor.  Thermocouples are almost always made out of copper.  Most of the time it is located in the front of the valve, but sometimes it can be in the rear.  The easiest way to find it is to take a flashlight and look for a tube that looks copper.  It will always connect back to the valve somewhere with a brass fitting.



Gas fireplaces do not have any power connected to the valve so there should be no worry about shocking yourself.  The only component that is plugged into power would be a the fan system which should not be in the way for testing the thermocouple.  Most of the time, you can always unplug the fan if you are worried about this.

Once you’ve located where the thermocouple connects to the main control valve, you need to disconnect it.

This is where you want to use your 7/16″ wrench (this is also where a short one comes in handy).  Simply loosen the fitting at the control valve and disconnect it from the valve.   You may have to gently bend it to get at the fitting.



Once you have it disconnected you will want to take your jumper wires and hook them onto the thermocouple.  You want to clip one onto the very tip and one somewhere on the copper tube like this:



Then we want to connect the other ends of the jumper wires on to the test leads of your meter like this:


Multimeter testing thermocouple


It does not matter which jumper wire goes on which testing lead.  If you have them backwards, when we get to testing,  it will just show a negative symbol before the voltage reading but the reading will be the same.

Next, we want to turn our meter on and turn it to the 2000m (millivolt) DC setting.  If you are using the Gardner Bender Digital Multimeter, it should look like this:


digital multimeter gas fireplace repair


If you are using another device, instead of the “V” symbol, it may say DC or have the same “solid line with dashed lines underneath” symbol.  Now that the meter is on and everything is connected we must now light the pilot.  Refer to the How to Light Your Pilot Light tutorial if you are unfamiliar with this.

Now we just light the pilot like normal only this time when the pilot ignites (because the thermocouple is disconnected) it will not stay lit on its own.  You must light it and continue to hold in the pilot knob while you read the voltage.

The voltage on the thermocouple should keep climbing until it reaches somewhere between 8-30 millivolts as shown above.

If you have a reading around 7 millivolts or below, you should change the thermocouple.

Need additional help with testing your thermocouple? Here is a short video tutorial from that goes over this process:


Shop for Thermocouples

I will post a tutorial on how to fix these problems shortly.  Thank you for visiting and please return!