AO Smith 2 Flashes: Low Thermopile Voltage

The pilot lights, but the flame dies a few seconds after you let go of the control knob, and the status light answers with two flashes. That's AO Smith's shorthand for a thermopile that isn't generating enough voltage to hold the gas valve open on its own.

Applies to: AO Smith gas storage tank water heaters with electronic ignition

What it means

Low Thermopile Voltage — the pilot's thermopile isn't generating enough voltage to keep the gas valve open, often experienced as "the pilot won't stay lit." Think of the thermopile as the pilot's power supply for the gas valve: it needs the pilot flame licking directly across its tip to generate a small, steady voltage. Anything that weakens that contact can trip this code even though the pilot itself lights just fine on its own.

Most common causes

Weak or misaligned pilot flame not fully contacting the thermopile
Dirty or corroded thermopile connection at the gas valve
Failing thermopile

What you can safely check yourself

Check that the pilot flame fully envelops the thermopile/thermocouple tip when lit
Clean the connection point at the gas valve if accessible

When to call a professional

The pilot flame looks correctly positioned but the code persists — the thermopile likely needs replacement.

Thermopile and gas valve connections should be tested and replaced by a licensed plumber or HVAC technician, not adjusted repeatedly by trial and error.

What the fix costs

Fixing a thermopile issue is on the cheaper end of water heater repairs: parts and labor for a thermopile replacement commonly run $75–$250, which lines up with the low end of the ranges Angi and HomeGuide publish for simple sensor-type fixes. A basic diagnostic visit, if the pilot alignment alone doesn't clear the code, is typically $150–$300. Treat both as national averages rather than a fixed price for your home.

Related codes

All AO Smith codes

Every AO Smith water heater fault code — the full list on one page. This is one specific cause behind a broader symptom — see pilot light won't light or won't stay lit for the full troubleshooting picture.

Safety. It's fine to look at the pilot flame and how it's positioned, but do not adjust, bypass, or wire around the thermopile or gas valve yourself. If the pilot won't light at all, or you smell gas while trying, stop and call a licensed technician rather than repeatedly re-lighting it.

Sources

AO Smith gas water heater status light / blinking fault code chart (manufacturer documentation)

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.