Before beginning this section make sure your pilot light is running, and you have read the Main Burner Troubleshooting Section so we can rule out faulty wall switch and bad pilot flame issues. If these issues have been ruled out, chances are your thermopile sensor is faulty. Just like a thermocouple, your thermopile will generate some voltage when heated up by the pilot flame. We can test the voltage that the thermopile is giving off by using our Digital Multimeter.
Time to Complete: 5 minutes
You will want to use your multimeter to test your thermopile leads. They are connected to the gas control valve, so the very first thing we should do is to locate the valve.
Here is a picture of your typical fireplace:
If you have grills or louvers on your fireplace It’s very common that the main control valve is typically located underneath the lower grill of the fireplace:
But control valves can be located in many different areas. If you are having trouble locating the main control valve, please leave a comment with a picture or make/model of the fireplace and I will help you locate it.
After you have found your main control valve, we have to locate the thermopile sensor. The 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.
The end of the thermopile wire branches off into two leads. These are usually red & white wires:
Another way to locate your thermopile leads is to look at your valve. Your thermopile should be hooked up to the TP and TP/TH terminals on the valve (the “TP” stands for Thermopile):
With the leads still attached to your valve, and the pilot light running, you can test the voltage on the terminals that the thermopile is connected to. (Again, there is less than 1 volt of electricity so there’s no danger of electrocuting yourself). To test voltage, you’ll want to put your Gardner Bender Digital Multimeter on the DC milivolt setting:
The DC setting will either say “DC” on your meter or have a symbol that looks like this: Then take your multi-meter leads and place one of them on the TP/TH terminal, and the other on the TP terminal (this is also where your thermopile should be connected). It does not matter which probe goes on which terminal:
You will need to make sure when you do this test that your pilot light has been running for around 2 minutes, AND make sure your wall switch, remote, or toggle switch that you use to turn the fireplace on is in the “off” position. With your multimeter leads on the TP and TP/TH terminals, it should be measuring at least 325 millivolts. If it is less than 325 millivolts, this means it is weak and you should replace it. Here we can see we are getting almost 850:
If you are reading above 325 millivolts on your meter, the next step would be to turn on your wall switch, remote, or toggle switch to attempt to ignite the main burner flame ( just like you would if you were turning on your fireplace). If everything is functioning properly, you should see the voltage reduce by a little more than half or at least 110mv:
What happens if the voltage does not reduce by a little more than half?
1.) THE VOLTAGE DROPS TO ZERO
When you turn the wall switch on, If your meter reads 0 or very low, you know you have a short in your thermopile and you should replace it.
2.) THE VOLTAGE DOES NOT CHANGE AT ALL
If the voltage doesn’t move at all, this means that the switch is not signaling the valve to turn on. First, make sure you are using the correct wall switch or toggle switch to ignite the fireplace. Next, find the wall switch wire coming into the fireplace from down below. It should look something like this:
Trace this back to the main control valve:
Check to see if the wire goes anywhere else before connecting to the valve. Sometimes it will run through an additional toggle switch that needs to be turned on or through a safety “snap-disc” interrupting the wall switch wiring. Also, you may want to make sure the wall switch wiring has not came disconnected from the “TH and TP/TH” terminals on the valve. For more on testing your switch circuit, please view the video below:
3.) THE VOLTAGE DROPS, BUT ONLY SLIGHLY
If the voltage barely changes, this is usually an indication that the main control valve itself is bad. Sometimes by tapping on the valve with a screwdriver you can get it go ignite. If this is the case, please visit the link below to take you to the next section on testing your main control valve:
Need additional help with testing your thermopile? Here is a short video tutorial from Fire-Parts.com that goes over this process:
A Note on “Snap discs” or “Limit Switches”
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.