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How to Replace a Gas Fireplace Thermocouple: Step-by-Step Guide
The thermocouple is the copper-lead sensor in your gas fireplace's pilot flame that keeps the gas valve's safety magnet open. When it wears out, the pilot lights while you hold the knob in but goes out when you let go. Gas fireplace thermocouple replacement is one of the simpler repairs: no gas line is opened, and the in-stock fireplace thermocouples we carry run $20 to $70. This guide also covers the thermopile, the two-wire part next to it that powers the burner.
Test Before You Buy
A thermocouple and a gas valve fail the same way, so check first. With the thermocouple disconnected from the valve and the pilot lit (knob held in), Heat & Glo and Empire both say to replace it if it reads less than 15 millivolts, and to replace the valve instead if it reads more than 15 mV and the pilot still will not stay lit. Our thermocouple and thermopile testing guide walks through every reading.
What a Thermocouple Does (and How It Differs from a Thermopile)
A thermocouple is two different metals joined at the tip. Heated by the pilot flame, it makes a small DC voltage that flows through the copper lead to the electromagnet inside the gas valve. Honeywell rates a new one at 26 to 32 mV. If the pilot goes out, the voltage falls, the magnet lets go and the valve shuts off the gas.
- Thermocouple: one copper tube with a threaded nut that screws into the valve. Its only job is to keep the pilot gas on while there is a flame.
- Thermopile: two wire leads with spade connectors on the valve's TP and TP/TH terminals. It makes hundreds of millivolts (replacements are rated 750 mV) and powers the main valve through the wall switch or remote.
Many fireplaces have both. SIT's 820 Nova Millivolt Plus valve, common in direct-vent fireplaces, uses a thermocouple for the safety magnet and a thermopile for the burner; the True Millivolt version and Robertshaw 700 series millivolt valves use only a thermopile. If the pilot stays lit but the burner will not come on, the thermopile is the likelier suspect.
Where Is the Thermocouple on a Gas Fireplace?
At the pilot, behind the lower louver or access panel. Follow the small metal tubes from the gas control valve to the pilot burner: the thermocouple tip and the thermopile sit right in the pilot flame, and Heat & Glo and Empire both say their tips should be covered with flame. The other end of the thermocouple screws into the side of the valve.
Tools and Materials
- The replacement thermocouple (same length and connector as the old one)
- A small open-end wrench that fits the nut (Robertshaw calls for a small 4 in wrench for the final quarter turn)
- A multimeter with a DC millivolt range, for testing
- Flashlight, and a phone to photograph the log layout and wire routing
Step 1: Find the Right Replacement
- Part number first. Your owner's manual or our model number guide gets you the original part number, which is the surest match.
- Length. Measure the old lead from tip to nut and buy the same length. Honeywell's Q390, for example, comes in 18, 24, 30 and 36 in leads.
- Connector. Most use a plain threaded nut (the SIT 820 Millivolt Plus takes an 11/32 in thermocouple fitting). Some fireplaces use a thermocouple with an interrupter block and two wire leads at the valve end; replace those with the same style. Vent-free valves for ODS pilots use an M8 thread, and the ODS thermocouple is replaced as part of the pilot assembly (see our ODS pilot guide).
- OEM or universal. A brand-specific part is made for your pilot bracket. A universal-style thermocouple comes with adapter fittings for the pilot burners its maker lists (Honeywell's Q390 adapter lists six Honeywell pilots), so check that yours is on the list.
Step 2: Shut Off the Gas and Let It Cool
- Turn the switch or remote off, then turn the gas control knob to OFF.
- Close the appliance shutoff valve (usually in the control compartment or on the wall nearby).
- Let the pilot area cool before you touch it.
- If you smell gas, stop. Do not touch electrical switches; leave and call your gas supplier from a neighbor's phone.
Step 3: Get to the Pilot and Valve
- Lower access panel or louver: held by screws, clips or a hinge on most units.
- Glass front: some direct-vent fireplaces need the glass removed to reach the pilot. Use the clips or latches your manual shows.
- Logs: photograph the log layout before you lift any logs out, so they go back the same way.
Step 4: Remove the Old Thermocouple
- At the valve: unscrew the thermocouple nut from the side of the valve body with the open-end wrench (not pliers, which round off the soft nut).
- At the pilot: note the exact position and angle of the tip in the pilot bracket, then release it from its clip or nut.
- Routing: note how the lead runs so the new one can follow the same path away from the burner.
Step 5: Install the New Thermocouple
- Seat the tip in the pilot bracket in the same spot as the old one. Honeywell wants the pilot flame to cover 3/8 to 1/2 in of the tip.
- Route the lead along the old path with gentle curves. Honeywell says not to bend it within 1/2 in of either end. Coil any extra length loosely near the valve.
- Connect it to the valve: start the nut by hand all the way in, then a quarter turn with the wrench. Heat & Glo, Empire, SIT, Robertshaw and Honeywell all give this same hand-tight-plus-1/4-turn instruction, and Robertshaw warns that overtightening can damage the thermocouple or the magnet. The connection must be clean and tight; it is an electrical joint.
Step 6: Light and Test
- Open the shutoff valve. Turn the knob to PILOT, press it in and light the pilot.
- Keep the knob pressed in: SIT says at least 30 seconds, Empire about one minute after the pilot lights.
- Release the knob. The pilot should stay lit. Empire suggests letting it burn another one to two minutes before you turn the knob to ON.
- Turn the knob to ON and run the burner from the switch or remote.
- If the pilot goes out, SIT says to wait at least five minutes before relighting so the safety interlock resets. If it will not stay lit after several tries, Empire says to turn the knob to OFF and call a service technician.
Optional checks: a new thermocouple should read 26 to 32 mV disconnected (Honeywell), and once the pilot is put out, pilot gas should stop within 2-1/2 minutes (Honeywell's safety shutdown check).
How to Replace a Gas Fireplace Thermopile
If the pilot stays lit but the burner will not come on, test the thermopile first: Heat & Glo wants at least 325 mV at the valve's TH-TP and TP terminals with the switch off, and more than 125 mV with the switch on (Empire: 350 mV and 200 mV). If it is low with a good pilot flame and clean connections:
- Turn the knob to OFF and let the pilot area cool.
- Pull the two thermopile leads off the TP and TP/TH terminals. Photograph or label them first.
- Release the thermopile from the pilot bracket the same way it is held (a nut or a clip).
- Insert the new thermopile fully into the pilot bracket; Heat & Glo lists a thermopile that is not fully inserted as a cause of low output.
- Reconnect the leads to the TPTH and TP terminals and make sure they are tight.
- Light the pilot and re-test before closing up.
Never put a jumper wire across the valve coil terminals while you work: SIT warns it will damage the millivolt operator.
If the Pilot Still Won't Stay Lit
- Check that the flame covers the tip; a weak, lazy or yellow pilot points to a dirty orifice or a worn pilot assembly.
- Recheck the nut at the valve: snug, not loose, not crushed.
- Measure the new thermocouple. If it reads above 15 mV and the pilot still drops out, Heat & Glo and Empire both say to replace the gas valve; see how to replace a gas fireplace gas valve.
- Maxitrol GV60 valves connect the thermocouple through an interrupter block; use our GV60 troubleshooting guide.
Common Mistakes to Avoid
- Buying the wrong length or connector. Measure, and match the end fitting.
- Overtightening the nut. Hand tight plus a quarter turn is the makers' spec.
- Kinking the lead. Bend it in gentle curves, and never within 1/2 in of either end (Honeywell).
When to Call a Professional
- You smell gas at any point
- The valve fittings are corroded or damaged, or the knob will not turn by hand
- A new thermocouple tests good and the pilot still will not hold
- Local code requires a licensed technician for gas appliance repairs
Thermocouple and Thermopile Parts
Match the length, connector and part number of the part you are replacing. Pilot assemblies come in natural gas (NG) and propane (LP) versions.
Shop by brand: all thermocouples and thermopiles · Heatilator parts.
Frequently Asked Questions
Can I replace a gas fireplace thermocouple myself?
Most handy homeowners can: it is an electrical connection at the valve and a clip or nut at the pilot, and no gas line is opened. Call a licensed technician if you smell gas, the fittings are damaged, or a new thermocouple does not fix it.
How much does it cost to replace a gas fireplace thermocouple?
The part is the main cost if you do it yourself: the in-stock fireplace thermocouples we carry run $20 to $70 depending on brand and length (Fireplace Choice prices, October 28, 2026). A technician adds a service call, so ask for a quote that includes testing the valve.
Are thermocouples universal?
Not fully. Universal-style thermocouples come with adapter fittings, but only for the pilot burners their maker lists, and valves differ: SIT's Millivolt Plus uses an 11/32 in fitting, ODS vent-free valves use an M8 thread, and some fireplaces need an interrupter block. Match the old part's length and both ends.
How do I clean a fireplace thermocouple?
Heat & Glo's maintenance list says to inspect the thermocouple, thermopile or IPI sensor rod for soot, corrosion and deterioration, and to clean it with emery cloth or replace it. Cleaning helps a sooted tip; a cracked or badly corroded one should be replaced.
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