Water Heater Thermocouple Problems
On this page (6 sections)
The Short Answer: On a standing-pilot gas water heater, the thermocouple sits in the pilot flame and tells the gas valve the pilot is lit. When it fails, the pilot lights while you hold the knob but goes out when you let go. Common causes are a worn thermocouple, a tip out of the flame, a dirty pilot, or a loose valve connection. If the thermocouple tests good, suspect the valve.
Does your heater even have a thermocouple?
Not every gas water heater uses one:
- Older and basic standing-pilot heaters: a single thin thermocouple lead running from the pilot to the gas valve. This page applies.
- Heaters with a status light on the gas control (for example Bradford White ICON, many A. O. Smith, State and Rheem models): these use a thermopile, which produces much more voltage to power an electronic control. A thermocouple will not work in its place. Weak thermopile output usually shows as a flash code; see Bradford White weak pilot signal.
- Electronic-ignition and power-vent heaters: use a flame sensor and igniter, not a thermocouple.
The rating plate, the lighting label, and the gas control itself tell you which you have. Photograph them before ordering any part.
Signs the thermocouple is the problem
| What happens | Likely cause |
|---|---|
| Pilot lights while holding the knob, goes out when released | Thermocouple, its position in the flame, or the gas valve magnet |
| Pilot holds only if you keep the knob down a long time | Weak thermocouple output or a weak pilot flame |
| Pilot stays lit for hours or days, then goes out | Draft, dirty pilot, intermittent thermocouple, or flame arrestor clogged |
| Pilot won’t light at all | Gas supply or igniter, not the thermocouple; see piezo igniter won’t spark |
| Pilot fine, main burner never comes on | Gas valve or thermopile; see pilot lit but burners won’t ignite |
| Water dripping onto the pilot area | Leak or condensation reaching the flame; see water dripping on the pilot |
What actually goes wrong
Worn thermocouple. The tip sits in a flame every hour of every day and eventually stops producing enough voltage to hold the valve open. This is the most common failure, and the part is inexpensive.
Tip out of the flame. The pilot flame should be steady and blue and wrap around the top of the thermocouple tip. If the bracket was bumped or the tip sits beside the flame, a good thermocouple can still fail to hold.
Weak or yellow pilot. A dirty pilot orifice or lint drawn into the burner area makes a small, lazy flame that does not heat the tip enough. Cleaning the pilot fixes this; a new thermocouple does not.
Connection at the gas valve. The thermocouple threads into the gas control. A connection that is loose, or one that was overtightened and damaged, can lose the small voltage before it reaches the valve.
Gas valve magnet. Inside the valve, the thermocouple’s voltage holds a small magnet open. If the thermocouple is producing normal output and the pilot still drops, the valve itself has failed and is replaced as a unit.
What you can check safely
- Follow the lighting instructions on the heater’s label exactly, including the waiting period after turning the knob off.
- Look through the sight glass at the pilot: is it blue and steady, and is it touching the thermocouple tip?
- Note whether the pilot goes out at certain times, such as when a bathroom fan, dryer or furnace runs, or on windy days.
- Look around the base for lint, dust or debris that could block air to the burner.
Do not loosen the thermocouple connection, the pilot tube, or any gas fitting, and do not remove the inner burner door on a sealed-combustion heater. If you smell gas, leave and call the gas utility from outside.
What the technician will check
A technician measures the thermocouple’s millivolt output with the pilot lit, both disconnected and connected to the valve. That separates a weak thermocouple from a failing valve magnet. They also clean the pilot, position the tip correctly, check draft and combustion air, and confirm the replacement thermocouple is the correct type and length for the valve. If the problem continues after a new thermocouple, see pilot won’t stay lit after replacing the thermocouple.
Frequently Asked Questions
Q: How long does a water heater thermocouple last? A: It varies with flame quality and conditions. Many last for years. A dirty or overly large pilot flame can shorten its life.
Q: Can a universal thermocouple fit my heater? A: Only on heaters designed for a standard thermocouple. It must be the right length and connection type for the valve. It will not work on heaters that use a thermopile or electronic ignition.
Q: If I replace the thermocouple and the pilot still goes out, what’s next? A: Check that the new tip is in the flame and the pilot is clean and blue. If both are fine, the gas valve is the likely cause, which is a technician repair.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- Bradford White: find your water heater manual (opens in a new tab)Use the model and serial number to locate the correct documentation.
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