Pilot Light Won't Stay Lit
Updated February 20, 2026
Lights but will not stay lit? Almost always the thermocouple -- a safety sensor that detects the pilot flame. When it fails, the gas valve shuts off. Straightforward DIY repair, under $25.
Overview
Small flame that ignites the main burner when the thermostat calls for heat. Will not stay lit after releasing the knob? Thermocouple. Thin metal rod in the pilot flame generates millivolts that signal the gas valve to stay open. Worn, corroded, or mispositioned? Not enough voltage, gas valve closes. Critical safety feature -- never bypass it. Clean or replace the thermocouple. Cheap part, any hardware store.
Symptoms
- Pilot lights when holding the knob but goes out within seconds after releasing
- Flame is small, weak, or yellow/orange instead of a strong blue cone -- dirty orifice or insufficient gas
- No hot water -- main burner cannot ignite without the pilot
- Faint gas odor near the heater after pilot goes out -- residual gas from the pilot tube
- Pilot going out intermittently over weeks before failing completely -- thermocouple gradually degrading
Common Causes
- Worn thermocouple -- most common cause. Tip oxidizes over 5-10 years and loses millivoltage. When voltage drops below the gas valve threshold, pilot goes out.
- Thermocouple mispositioned -- tip must be in the upper, hottest portion of the pilot flame. Bumped or shifted during maintenance? Not hot enough even if the part is still good.
- Dirty pilot orifice -- tiny brass nozzle clogged with dust, debris, spider webs, or corrosion. Weak, small flame that is yellow or orange instead of blue. Does not heat the thermocouple adequately.
- Drafts -- near exterior doors, windows, dryer vents, or HVAC returns. Blows the pilot out. More common on older open-combustion heaters.
- Faulty gas control valve -- rare. Thermocouple tests good, flame is strong, still will not stay lit. Gas valve has failed internally. Plumber job.
What You'll Need
How to Fix It
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Relight the Pilot and Observe
Gas knob to OFF, wait 5 minutes. Knob to PILOT, press and hold. Igniter button or long match. Hold 60 full seconds -- thermocouple needs time to heat up. Slowly release. Stays lit? One-time event. Goes out immediately? Thermocouple.
Warning: Strong gas smell? Do not light anything. Leave immediately, no electrical switches. Call your gas utility's emergency line from outside. -
Inspect the Pilot Flame and Thermocouple Position
While holding the knob, observe the flame. Healthy: steady blue cone, 1 inch tall, engulfing the upper third of the thermocouple. Weak, yellow, or not reaching the tip? Pilot orifice or positioning issue, not necessarily the thermocouple. Tip should be about 1/4 inch into the flame.
Tip: Strong blue flame but thermocouple tip is not in it? Gently bend the thermocouple toward the flame (gas off). Designed to be slightly flexible for this adjustment. -
Clean the Pilot Orifice and Thermocouple
Gas off. Access panel off. Compressed air blast on the pilot orifice to dislodge dust. Fine sandpaper (400-600) or emery cloth on the thermocouple tip to remove oxidation and soot. Soft brush around the orifice. Reassemble, relight, test. Cleaning alone fixes many cases.
Tip: Pencil eraser works for the thermocouple tip in a pinch. Mild abrasive removes oxidation without damage. -
Replace the Thermocouple
Gas off, let it cool. Single nut connects the thermocouple to the gas valve (7/16 or 3/8 inch). Unscrew, remove from the bracket. Take old one to match length and connector. Universal types in 24 or 30 inch, $8-20. New tip into the bracket (in the flame path), thread the nut in. Quarter turn past hand-tight.
Warning: Snug only. Overtightening cracks the gas valve fitting -- much more expensive repair. Hand-tight plus a gentle snug with a wrench. -
Test After Replacement
PILOT, press and hold, light. Hold 30-60 seconds, release. Stays lit? Turn to your temperature setting. Burner should fire when water is below the set point. Steady blue flame. Monitor for 24 hours.
Tip: Soapy water spray on the thermocouple connection. Bubbles mean a gas leak -- tighten the connection. -
Address Draft Issues (If Pilot Keeps Going Out)
New thermocouple works but pilot goes out intermittently? Drafts. Check nearby doors, windows, dryer vents, HVAC returns. Access panel properly seated? High-draft area? Draft shield or relocate. Newer heaters with sealed combustion and electronic ignition eliminate this entirely.
Tip: Replacing an older atmospheric-vent heater? Power-vent or direct-vent models use electronic ignition. No standing pilot, no pilot problems, better efficiency.
When to Call a Pro
Call a plumber or gas tech if you smell gas, if thermocouple replacement does not fix it (gas valve may need replacing), if you are not comfortable with gas appliances, if the heater is also making unusual sounds, or if it is over 10-12 years old with multiple failures. Gas valve replacement is always a professional job.
Prevention Tips
- Replace thermocouple every 5-7 years proactively. Inexpensive, and failure means no hot water.
- Keep the area clear of dust, debris, clutter. Reduces orifice clogging and fire risk.
- Annual check: pilot flame should be a steady blue cone engulfing the thermocouple tip.
- Minimize drafts near the heater. Doors and windows closed, burner compartment cover in place.
- Vacuum the base and around the access panel annually. Prevents dust from entering the combustion chamber.
- Garage or utility room? Combustion air vents (louvered openings) must not be blocked. Burner needs adequate air.