Water Heater Pilot Light Won't Stay Lit — Fixes
On a gas water heater, if the pilot lights but goes out when you release the knob — or keeps going out — the cause is almost always the thermocouple or a related safety device. Here's how to diagnose it.
1. Bad Thermocouple (Most Common)
The thermocouple is a sensor whose tip sits in the pilot flame. It generates a small voltage that tells the gas valve "the pilot is lit, keep gas flowing." When it weakens or fails, the valve shuts the gas off and the pilot dies seconds after you release the knob. A thermocouple is cheap and a common DIY replacement — the #1 cause of this exact symptom.
2. Dirty or Misaligned Pilot/Thermocouple
A dirty pilot orifice gives a weak flame that doesn't fully heat the thermocouple tip. Clean the pilot assembly, and make sure the thermocouple tip is properly positioned in the flame. A weak, yellow, or floppy pilot flame is a clue.
3. Tripped Thermal Switch (FV Sensor)
Many newer heaters have a thermal switch on the side of the burner chamber (part of the flammable-vapor-ignition-resistant design). If it trips — often from lint/dust blocking the air intake screen at the base — the pilot won't stay lit. Vacuum the intake screen/arrestor at the bottom of the heater; a tripped switch may need replacement.
4. Gas Supply or Valve
Confirm gas is on and other gas appliances work. If the thermocouple, pilot, and thermal switch are all good but the pilot still won't hold, the gas control valve itself may be failing — a bigger repair.
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FAQ
Almost always a failing thermocouple — the sensor that keeps the gas valve open. It's cheap and a common DIY replacement. A dirty pilot or a tripped thermal switch (from a clogged air intake) can also cause it.
Usually $10-20 for the part, and it's a common DIY repair. Replace it before assuming the more expensive gas valve has failed.
Always unplug an appliance and shut off its water supply before servicing. This guide is informational and not a substitute for a qualified technician.