A.O. Smith Water Heater E4 — Thermopile Fix

What Does This Error Mean?

E4 means the thermopile is not generating enough voltage to keep the gas valve open. Clean the thermopile tip first — carbon buildup is the most common cause. Replace the thermopile if it still reads below 400 mV after cleaning.

Affected Models

A.O. Smith Signature Gas
A.O. Smith ProLine Gas
A.O. Smith GPVH Series
A.O. Smith Natural Gas Tank Models

Common Causes

Carbon deposits on thermopile tip reducing output voltage
Thermopile near end of service life
Weak or misaligned pilot flame not fully heating the thermopile
Loose or corroded wire connections at the gas valve

How to Fix It

Confirm the pilot flame is healthy.

A healthy pilot flame is blue and wraps fully around the thermopile tip. A small, yellow, or flickering pilot does not heat the thermopile adequately and can cause E4 without the thermopile being faulty.
Clean the thermopile tip.

Turn off the gas supply and let the unit cool. Gently clean the thermopile tip — the rod positioned in the pilot flame — with fine steel wool. Carbon and oxide buildup dramatically reduce voltage output.
Check the thermopile wiring.

Trace the two thermopile wires to the gas valve connectors. Ensure both are tight and the connector pins are free of corrosion. Corroded pins add resistance and reduce effective voltage.
Test the thermopile voltage.

With the pilot lit, measure DC millivolts across the thermopile terminals using a multimeter. A healthy thermopile reads 650–850 mV. Below 400 mV means it should be replaced.

When to Call a Professional

If replacing the thermopile does not resolve E4, the gas valve itself may have failed — gas valve replacement requires a licensed plumber.

Frequently Asked Questions

What is the difference between E1 and E4 on an A.O. Smith?

E1 is ignition lockout — the burner failed to ignite. E4 is a thermopile voltage fault — the pilot is lit but the thermopile cannot generate enough power to open the main gas valve.

Is A.O. Smith E4 covered by warranty?

E4 from a manufacturing defect in the thermopile is generally covered under warranty. Normal wear on an older unit typically is not. Contact A.O. Smith with your model and serial number to check.