The thermocouple is a small sensor that sits in the pilot flame. When hot, it generates a tiny millivolt current that signals the gas control valve to keep the main gas supply open. When the thermocouple fails (or the pilot flame isn't fully on it), the gas valve closes and the pilot light extinguishes. Result: "I lit the pilot but it won't stay lit." Thermocouple replacement fixes this in 80% of cases.
Rheem thermocouple part number
Standard Rheem residential thermocouple: SP12090A, ~$25–$40. Universal-compatible 24" thermocouples from any plumbing supplier also work — they all use the same 11/32" or 7/16" connection at the gas valve.
Some FVIR-compliant Rheem units (Performance Platinum and post-2015 Performance Plus) use a thermopile or "powerpile" instead — looks similar but has a longer wire and produces higher voltage. Verify with the data plate or photograph the existing component before ordering.
How to confirm the thermocouple is the failure
Symptom check:
- Press and hold the pilot button while lighting the pilot; pilot lights normally
- Hold the button for 30+ seconds after the flame is steady
- Release the button — pilot extinguishes immediately or within 5 seconds
- This sequence = thermocouple failure (or, less often, gas control valve magnet failure)
If the pilot stays lit while you hold the button for 60+ seconds but goes out as soon as you release, it's the thermocouple 80% of the time and the gas control valve 20%.
Replacement procedure
- Shut off the gas at the supply valve to the water heater
- Wait 5 minutes for gas in the manifold to vent
- Open the burner access door (usually held by a single screw or sliding latch on the front of the unit)
- Locate the thermocouple — copper tube running from the gas control valve down into the burner chamber, with the sensing tip in the pilot flame area
- Disconnect at the gas valve — 11/32" or 7/16" small nut. Loosen by hand if possible (these are not torqued tight).
- Remove the thermocouple bracket inside the burner chamber — usually a small clip or one screw. The thermocouple slides out from the bracket.
- Install the new thermocouple: feed the tip into the bracket so the tip sits directly in the pilot flame path (not above or beside it)
- Connect the other end to the gas valve — hand-tight first, then 1/4 turn with a wrench. Do not over-torque; you'll deform the soft copper and create a leak.
- Close the burner access, restore gas supply
- Relight the pilot following the lighting instructions on the unit (press button, light, hold for 30 seconds, release)
- Pilot should now stay lit — listen for the click of the gas valve magnet seating after 30–60 seconds
If it still won't stay lit
Failed thermocouple isn't the only cause. Other possibilities:
- Pilot flame too small: debris in the pilot tube. Use compressed air or a thin wire to clean the pilot orifice. Some Rheem pilots have an adjustment screw — verify pilot flame fully envelops the thermocouple tip.
- Gas control valve magnet failed: rare but possible. The magnet inside the valve that holds the gas open with the thermocouple's millivolt signal can fail. Replacement of the gas valve is required ($180–$280, pro recommended).
- Air in the gas line: after gas-supply work, air in the line will extinguish the pilot. Light the pilot 2–3 times to purge air; should stabilize after that.
- Spider webs in the burner chamber: common with units that haven't been cleaned. Vacuum the burner chamber gently with the access door open.
Common mistakes
Over-tightening the nut at the gas valve: deforms the copper end and creates a leak. Hand-tight + 1/4 turn is sufficient.
Bending the thermocouple sharply: the copper cracks at sharp bends, breaking the internal wire. Gentle curves only.
Mis-positioning the sensing tip: the tip must be directly in the pilot flame. If it's beside the flame (not in it), the thermocouple won't heat up enough to hold the gas valve open. Verify visually after the pilot is lit.
Bottom line
Thermocouple replacement is the most common Rheem gas service item — $25–$40 part, 30-minute job, fixes the "pilot won't stay lit" symptom 80% of the time. For the rare cases it doesn't, see our troubleshooting hub or check the gas valve page. For full parts directory see our Rheem parts hub.