If your water heater pilot light keeps going out, the thermocouple is the most likely culprit. It’s a cheap part and replacing it is the most common fix. That said, a bad thermocouple isn’t the only cause. A weak gas supply or a draft near the unit can produce the exact same symptom.
Why the Pilot Keeps Dying: Causes by Likelihood
Thermocouple failure (most common)
The thermocouple is a small metal rod that sits in the pilot flame. When the flame heats it, it generates a tiny electrical signal that tells the gas valve “pilot is on, stay open.” When the thermocouple wears out or gets dirty, that signal gets too weak. The gas valve reads it as “no flame” and shuts off the gas. Pilot goes out.
Signs pointing to the thermocouple: the pilot lights fine, burns for 30-60 seconds, then dies when you release the button. Or it relights easily every time but never stays. The thermocouple tip may look coated in buildup or positioned slightly off the flame.
Draft or combustion air issue
Gas water heaters need combustion air. If the unit is in a closet, utility room, or confined space without adequate ventilation, the pilot can starve. A nearby exhaust fan, a door that gets opened and closed, or a gap under a door can all create a draft that snuffs the flame.
Signs pointing to a draft: the pilot goes out unpredictably, more often when certain appliances run or when doors are open. The flame looks unstable or flickering even when lit.
Gas supply problem
If gas pressure to the unit is too low, the pilot flame will be weak and may not generate enough heat to satisfy the thermocouple. A partially closed shutoff valve, a failing gas regulator, or supply issues affecting the whole house can all contribute.
Signs pointing to gas supply: other gas appliances (furnace, stove) also seem weaker than normal. The pilot flame is unusually small even when it does light.
Dirty pilot orifice
The small opening the pilot gas flows through can clog with mineral dust or debris over years. The flame comes out small and off-angle, doesn’t hit the thermocouple properly, and the unit shuts off.
How a Tech Diagnoses It
A water heater tech will check these in roughly this order:
- Visually inspect the thermocouple tip and position. If it’s not sitting in the hottest part of the pilot flame, that alone can cause the problem.
- Test the thermocouple with a multimeter. A working thermocouple in full flame typically generates somewhere in the range of 20-30 millivolts, though acceptable minimums vary by gas valve manufacturer. A consistently low reading compared to that range points to a weak or failing thermocouple and replacement is the likely next step.
- Check the gas valve. Sometimes the valve itself is faulty and won’t hold even with a good thermocouple signal. This is less common but happens on older units.
- Inspect the flue and combustion air supply. On power-vent or direct-vent units, a blocked or improperly connected vent pipe can cause pressure issues that affect the pilot.
- Check gas pressure at the unit with a manometer. Standard residential natural gas delivery runs around 7 inches of water column. Anything significantly lower points to a problem upstream.
What You Can Do
Relighting the pilot following the manufacturer’s label instructions is safe for most homeowners. The procedure is printed on the unit. It typically involves turning the gas control knob to PILOT, holding it down, pressing the igniter or using a long lighter, and holding for 30-60 seconds before releasing.
If that works but the pilot dies again within a day or two, note the pattern and call a plumber. That symptom description is enough for a tech to narrow the diagnosis fast. Everything beyond relighting, including cleaning or replacing the thermocouple, touching the gas valve, or inspecting the vent, involves gas system components and needs a licensed plumber.
When to Call a Licensed Plumber
Call a professional if:
- You smell gas anywhere near the unit, even faintly. Don’t relight, don’t flip switches. Get out and call the gas company first, then a plumber.
- The pilot won’t stay lit after relighting it per the label. The thermocouple or gas valve likely needs service.
- Your unit is older than 10-12 years and this is the second or third component failure. You may be looking at a replacement conversation, not another repair.
- The vent flue is involved. Blocked or disconnected vent pipes can cause carbon monoxide buildup in the home.
- You’re not sure what you’re looking at. A pilot light diagnosis takes a tech about 15 minutes.
This site doesn’t offer plumbing services. When you’re ready to hire someone, look for a licensed plumber and verify their license at cslb.ca.gov before they start work. A valid C-36 (Plumbing) license is what you want to see in California. Get at least two quotes for any repair.
The thermocouple really is the answer most of the time. It’s a cheap part that fails on a schedule. Once you’ve confirmed the symptom, let a licensed plumber handle the repair. Gas components aren’t the place to wing it.