How to Replace a Thermocouple
Updated February 20, 2026
Fix a pilot light that will not stay lit by replacing the thermocouple -- the $10 safety sensor that tells the gas valve to stay open. 15-minute job.
Overview
Pilot lights but goes out when you release the button? That is the thermocouple. It is a thin metal rod next to the pilot flame that generates a tiny voltage when heated. That voltage holds the gas valve open. When the thermocouple fails, no voltage, gas valve closes, pilot goes out. Replacing it is one of the simplest water heater repairs. Unscrew the old one, screw in the new one, relight the pilot. Universal thermocouples ($8-15) fit most standard gas water heaters. Takes 15 minutes.
What You'll Need
Safety First
- Turn the gas valve to OFF and wait 5 minutes before working near the burner assembly. Let any residual gas dissipate.
- Do not overtighten the thermocouple fitting at the gas valve. It is a soft copper or brass connection -- finger-tight plus a quarter turn with a wrench is enough. Overtightening strips the threads.
Step-by-Step Instructions
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Turn Off the Gas and Locate the Thermocouple
Turn the gas control valve to OFF. Wait 5 minutes. Remove the burner access panel at the bottom of the water heater. Look at the pilot assembly -- you will see the pilot tube (small gas line), the thermocouple (thin copper rod with a bulb tip), and possibly a flame sensor. The thermocouple tip sits in or right next to the pilot flame. Follow it back to where it connects to the gas control valve -- a small nut threaded into the valve body.
Tip: Take a photo of the assembly before disconnecting anything. You need to route the new thermocouple the same way and position the tip in the same spot. -
Disconnect the Old Thermocouple
At the gas control valve: use a 7/16 inch wrench (or adjustable wrench) to unscrew the thermocouple nut. Turn counterclockwise. It should come loose easily. At the pilot assembly: the thermocouple tip is held in a bracket or clip. Slide or unclip it. Pull the entire thermocouple out, routing it the same path it was installed.
Tip: If the nut is stuck, do not force it with a large wrench. Apply a drop of penetrating oil, wait 5 minutes, try again. Stripping the threads on the gas valve creates a much bigger problem. -
Install the New Thermocouple
Universal thermocouples come in 18, 24, and 30 inch lengths. Match the old one or go slightly longer (excess coils behind the panel). Insert the tip into the pilot assembly bracket -- the bulb should be in the same position as the old one, directly in the pilot flame path. Route the copper tube along the same path. Thread the nut into the gas valve by hand first, then snug with a wrench -- finger-tight plus a quarter turn.
Tip: The thermocouple tip must be positioned IN the pilot flame, not near it. If the tip is too far from the flame, it will not get hot enough to generate the voltage needed to hold the gas valve open. This is the most common installation mistake. -
Relight the Pilot and Test
Turn the gas valve to PILOT. Press and hold the knob. Ignite the pilot (electronic igniter or long lighter). Hold for 60 seconds to let the new thermocouple heat up. Release. The pilot should stay lit. If it does, turn the knob to ON. The main burner should ignite. Watch for 5 minutes to confirm the pilot stays lit consistently.
Tip: New thermocouple but pilot still goes out? Hold the button longer -- 90 seconds. A new thermocouple needs time to heat up on the first lighting. If it still fails after 90 seconds, the gas valve itself may be faulty. -
Check for Gas Leaks
Brush soapy water on the thermocouple connection at the gas valve. Watch for bubbles for 30 seconds. Any bubbles mean a gas leak -- tighten the nut slightly and retest. Replace the burner access panel. Monitor the water heater for the next few hours to confirm the pilot stays lit.
Tip: If you smell gas at any point during or after the repair, turn the gas valve to OFF immediately and call your gas company. Do not try to find the leak with a flame.
Pro Tips
- Buy a universal thermocouple kit that includes multiple tip adapters. Covers most brands without needing to identify the exact model.
- If the pilot flame is yellow or weak instead of a crisp blue, the pilot tube orifice is clogged. Clean it with compressed air or a fine needle while you have the assembly apart.
- Thermocouples degrade gradually. If the pilot has been going out intermittently before failing completely, that is normal thermocouple end-of-life behavior.
- Some newer water heaters use a thermopile instead of a thermocouple. A thermopile has two wires instead of one and generates more voltage for electronic gas valves. Make sure you buy the right type.
- Write the date of replacement on a piece of tape stuck to the water heater. Thermocouples typically last 5-10 years.
When to Call a Pro
Call a plumber or gas technician if the gas valve connection is stripped or will not stop leaking, if the new thermocouple does not fix the problem (gas valve may need replacement), if you have a sealed combustion or power-vent water heater with a complex burner assembly, or if you smell gas and cannot identify the source.