Time to Complete: 5 minutes
So you’ve bypassed the wall switch, the pilot flame looks good, and the main burner still wont come on. Chances are, your thermopile sensor is faulty. Just like your thermocouple, your thermopile will generate voltage when heated up by the pilot flame. We can test the voltage that the thermopile is giving off with our Digital Multimeter.
You will want to test your thermopile leads. They are connected to the gas control valve so first we should locate the valve.
The main control valve is typically located underneath the lower louver or grill of the fireplace:
Your 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 terminals on the valve:
With the leads still attached to your valve, you can test the voltage on these (again, there is less than 1 volt of electricity so there’s no danger of electrocuting yourself). You’ll want to put your Gardner Bender Digital Multimeter on the milivolt setting:
Then take your multimeter leads and place them on your sensor leads. It does not matter which one goes on which:
With your pilot light running and your wall switch or remote in the “off” position, you should be getting at least 300-350 millivolts. If it is less than that you may want to think about replacing it. Here we can see we are getting almost 850:
Next, turn your wall switch or remote on, just like you would if you were turning on your fireplace. You should see the voltage drop roughly in half:
What does this mean?
You should see the voltage drop roughly IN HALF:
1.)If it zero’s out, or goes very low, you know you have a short in your thermopile and you should replace it.
2.) If it doesn’t move at all, you may want to double check your wall switch or the wiring to it.
3.) If the voltage only drops a little bit, this may be an indication that the valve is bad.
Try tapping on the valve with a screwdriver. There will be more on this in the next section.
A Note on “Snap discs”
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.