AO Smith Water Heater Pilot Won't Stay Lit

What Does This Error Mean?

If your AO Smith pilot lights but goes out as soon as you release the gas valve button, the thermocouple has failed. A new thermocouple costs around $10–$20 and is one of the most common water heater repairs.

Affected Models

AO Smith ProMax
AO Smith Signature
AO Smith Signature Premier
AO Smith Effex
AO Smith Vertex

Common Causes

Failed or weak thermocouple (most common cause)
Dirty thermocouple tip not generating enough voltage
Pilot tube blocked by dust producing a weak flame
Failed gas control valve
Air in the gas line on a recent installation

How to Fix It

Verify the gas supply is on.

Check the gas shutoff valve at the heater and at the meter. If other gas appliances work, gas is not the issue. If they do not, contact your gas utility — there may be a meter or supply problem.
Hold the pilot button longer.

When relighting the pilot, hold the gas control button for at least 60 seconds after lighting. This gives the thermocouple time to heat up and generate enough voltage to keep the gas valve open. Releasing too early causes the flame to go out.
Clean the thermocouple tip.

Turn off the gas supply. Gently rub the thermocouple tip with fine emery cloth to remove oxide buildup. Reposition it so the pilot flame envelops the tip when lit.
Replace the thermocouple.

Turn off the gas at the heater shutoff. Unscrew the old thermocouple from the gas valve and the pilot bracket. Install a new thermocouple, ensuring the tip sits in the pilot flame. Relight the pilot per the unit's instructions.

When to Call a Professional

Thermocouple replacement is a DIY-feasible task on traditional AO Smith units. If you are uncomfortable working with gas, or if a new thermocouple does not fix the issue, contact a plumber or gas technician.