Pilot Light Won't Light or Won't Stay Lit

This applies to standing-pilot and electronic-ignition gas tank water heaters: the pilot either won't catch when you try to light it, or it lights and then goes out again once you release the control knob.

Most likely causes, ranked

• Weak or misaligned pilot flame. If the flame isn't fully wrapping the thermocouple or thermopile tip, that sensor never generates enough signal to hold the gas valve open, and the pilot dies the moment you let go of the knob. On AO Smith electronic-ignition tank models this is documented directly as AO Smith 2 Flashes — Low Thermopile Voltage.
• Worn-out thermocouple or thermopile. Even with a good flame position, the sensor itself degrades over years of heat cycling and eventually stops producing enough voltage.
• Air in the gas line. Common right after installation, a tank swap, or any work on the gas line — it can take several lighting attempts for trapped air to clear.
• Dirty or clogged pilot orifice or tubing. Dust, lint, or spider webs in the small pilot orifice restrict gas flow enough that the flame is too small or the wrong shape to hold.
• Gas control valve fault. Less common, but a failing valve can prevent the pilot circuit from working correctly regardless of thermocouple condition — see AO Smith 7 Flashes.
• Flammable vapor sensor lockout. On FVIR-equipped tank models, gasoline, solvent, or aerosol vapors near the unit can trip a safety lockout that blocks gas to the pilot entirely until it's cleared — AO Smith 6 Flashes.

Check this first

1. Follow the exact lighting sequence printed on the label inside the water heater's access panel — most models require holding the control knob in for 30–60 seconds after the pilot catches before releasing it, and releasing too early is the most common reason a pilot "won't stay lit."
2. Watch where the flame actually sits — it should fully surround the thermocouple or thermopile tip, not just brush past it.
3. Remove any gasoline, paint, solvents, or aerosol cans stored near the water heater and ventilate the area for several minutes before trying again — this clears a flammable-vapor lockout on FVIR models.
4. If the unit is newly installed or the gas line was recently worked on, try the lighting sequence two or three times to let trapped air bleed out.
5. Check that the pilot tubing and orifice aren't visibly dusty or obstructed (do this with the gas off).

Which fault code are you seeing?

If a code or blink pattern is showing, look it up directly: code lookup, or jump to a brand: Rinnai, Navien, Rheem, AO Smith.