Troubleshooting guide
Thermocouple vs. thermopile: what's the difference?
Both sit in the pilot flame and turn heat into a tiny bit of electricity. They do different jobs, and they fail in different ways.
The short version
Thermocouple
The safety part. It holds the gas valve open only while the pilot is lit. When it fails, the pilot won't stay lit.
Thermopile
The power part. It's a bundle of thermocouples (Empire describes it as many thermocouples in series) that makes enough voltage to open the main valve from a wall switch or remote. When it's weak, the pilot stays lit but the burner won't come on, or it comes on and drops out.
How a tech tests them
With a millivolt meter and the pilot lit. Typical figures from manufacturer service documents:
- Thermocouple: Empire's service manual expects about 18–30 mV open-circuit. Valor's GV60 guide treats about 12 mV as a good reading on its system and 9 mV or lower as likely defective. Different systems, different numbers.
- Thermopile: Empire says its thermopiles can produce up to 750 mV, and a minimum of about 325 mV open-circuit is generally acceptable.
A low reading isn't always a bad part. Manufacturers say to check the pilot flame first: if the flame isn't hitting the part properly, a new one won't help.
Can I replace one myself?
These parts connect to the gas valve, and the wrong part or an over-tightened fitting can damage the valve. Our advice: get the exact part number from your model's parts list, and have a qualified tech install it if you're not experienced with gas appliances. A parts match from a video call can save you a wasted order.
Common questions
What's the difference between a thermocouple and a thermopile?
A thermocouple is a safety device that holds the gas valve open only while the pilot is lit. A thermopile is several thermocouples combined to make more voltage, enough to open the main burner valve from a wall switch or remote.
How many millivolts should a fireplace thermopile put out?
It depends on the manufacturer. Empire's service manual, for example, says a minimum of about 325 mV open-circuit is generally acceptable for its thermopiles, which can produce up to 750 mV. Use your own manual's figure.
How do I know if my thermocouple is bad?
The classic sign is a pilot that lights but goes out when you release the knob, even though the pilot flame looks good and wraps the thermocouple tip. A millivolt test confirms it.
Related guides
Pilot lights, then goes out
It lights, but won't stay lit. Usually the flame isn't heating the sensor enough.
Read the guidePilot is lit, burner won't come on
The pilot's burning but the main flame never comes up.
Read the guidePilot won't light
Clicking but no flame, or no spark at all. The usual causes, in order.
Read the guideSources
- Empire Comfort Systems service manual: thermocouples, thermopiles and millivolt testing
- Valor (Miles Industries) Troubleshooting Handbook, Oct 2025 (GV60 control system)
Brand names belong to their owners. Fireplace Helpline isn't affiliated with or endorsed by any manufacturer. Steps and codes vary by model and year, so always check your own owner's manual.