AO Smith Water Heater E2 Error — Overheating Fix Guide

What Does This Error Mean?

E2 means the water in the tank has reached the high-temperature limit.
Either the temperature sensor reads high (overheat condition), or the high-limit safety switch has tripped.
This is a serious code — overheating water can scald and pressure can damage the tank.
Don't reset and run repeatedly without diagnosis.
Call a licensed plumber the same day.

Affected Models

AO Smith Vertex
AO Smith Voltex hybrid
AO Smith Signature Premier
AO Smith ProLine XE

Common Causes

Thermostat stuck in the closed position (heating non-stop)
Temperature sensor failed reading high
High-limit safety switch tripped (resettable manually)
Heating element relay welded shut (electric models)
Gas valve stuck open (gas models)

How to Fix It

Cut power or gas to the heater.

On electric models, turn the breaker off.
On gas models, turn the gas valve to off.
This stops further heating immediately.
Don't try to reset until power/gas is off and the water has cooled to a safe temperature.
Wait for water temperature to drop.

Run a hot tap somewhere in the house to draw water through the tank.
This dumps the overheated water and lets cooler water in.
Keep running until the tap water is just warm, not scalding.
Then close the tap.
Locate the high-limit reset (electric only).

On electric AO Smith heaters, there's a small red button on the upper thermostat behind the access panel.
That's the high-limit reset.
Pop the cover off (power off!) and press the red button until you feel a click.
If it didn't reset before, that's why the unit was stuck on E2.
Restore power or gas.

Turn the breaker or gas valve back on.
Watch the unit during the next heating cycle — does it stop at the set temperature or keep going?
If the heat doesn't shut off at the set point, the thermostat is bad and needs replacement immediately.
Call a plumber if E2 returns.

Repeated E2 codes mean a real safety issue.
Don't keep resetting and running.
The combination of heating element + thermostat + high-limit needs to be tested by a plumber with a meter.
Replacement parts are 30–80 USD; getting it right is the licensed installer's job.

Frequently Asked Questions

Can E2 cause the tank to explode?

In the worst case, yes — though modern tanks have multiple safety layers.
The temperature/pressure (T&P) relief valve will open before the tank ruptures, so you'd see hot water running out of that valve onto the floor as a warning sign.
Don't ignore E2 thinking it'll resolve itself.

Is E2 covered under warranty?

If the heater is under warranty and a thermostat or sensor failed, yes — AO Smith covers it.
You'll need a licensed plumber's diagnosis report.
If the issue is sediment buildup interfering with the sensor (common in hard water areas), warranty may not apply — flushing is owner maintenance.