Replace a Thermocouple (Standing Pilot)

Replace the thermocouple on a standing-pilot system: the tip sits IN the pilot flame and makes a small DC millivolt signal (healthy ~25–30 mV open-circuit; the gas-valve magnet drops out when it falls too low). Mount finger-tight plus about a quarter turn (do NOT overtighten), position the tip in the flame, and verify the pilot stays lit after you release the valve knob.

Approx. 30 min

Before you start

•Confirmed standing-pilot system (constant pilot + millivolt safety magnet), not an electronic-ignition unit — and the pilot won't stay lit after releasing the knob, which points at the thermocouple (or a fouled pilot/weak flame).
•Rule out a dirty/misadjusted pilot first — a thermocouple only works if the pilot flame actually envelops the tip; clean/adjust the pilot before condemning the thermocouple.
•Correct thermocouple length for the burner (common lengths ~18/24/30/36 in) with the matching connector nut for the gas valve.

Tools & materials

Open-end wrench (thermocouple nut) — do not overtighten · Multimeter (DC millivolts) and/or a thermocouple adapter/tester · Nut drivers + screwdrivers · Replacement thermocouple (correct length)

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Shut off & remove

Turn the gas to OFF and let it cool
Set the gas-valve knob to OFF (pilot out) and let the assembly cool before working. Note how the old thermocouple tip is positioned in the pilot flame so you can match it.
⚠ Gas valve OFF before disconnecting; let hot parts cool to avoid burns.
Unthread the thermocouple from the valve + bracket
Back the thermocouple connector nut off the gas valve (it's a slip nut over the copper lead) and free the tip from its pilot bracket. Inspect the pilot orifice while it's apart.
Tools: Open-end wrench

Mount the new thermocouple

Position the tip in the pilot flame
Seat the new thermocouple in the pilot bracket so the tip sits in the flame the same depth the old one did — roughly the top third of the tip should be enveloped by the steady inner cone of the pilot. A tip outside the flame never makes enough millivolts.
Target: Tip enveloped by the pilot flame (about the top 1/3 of the tip in the flame)
Tighten finger-tight plus about a quarter turn
Thread the connector nut into the gas valve by hand, then snug it only about a quarter turn with a wrench. OVERTIGHTENING crushes the contact and is the #1 way to ruin a new thermocouple — it must make a clean, low-resistance electrical contact, not be wrenched down.
Tools: Open-end wrench
Target: Finger-tight + ~1/4 turn — do NOT overtighten
⚠ Overtightening the connector kills the millivolt signal — snug only.

Light & verify

Light the pilot and read open-circuit millivolts
Follow the lighting sequence: hold the knob to PILOT, light it, hold ~30–60 s for the tip to heat, then read the output. A healthy thermocouple makes about 25–30 mV open-circuit (less under load through the valve magnet). A low/dropping reading means a weak pilot, bad position, or a bad thermocouple.
Tools: Multimeter (DC millivolts)
Target: ~25–30 mV open-circuit when hot; the valve magnet drops out below roughly half that
Release the knob and confirm the pilot holds
Release the gas-valve knob. The pilot must stay lit on its own — the thermocouple is now holding the safety magnet open. If it drops out, re-check flame position, the connection tightness, and the millivolt reading.
Target: Pilot stays lit after releasing the knob; main burner then lights on a call