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E3

AO Smith Water Heater

Severity: Moderate

Source: OnlyErrorCodes.com — Every error code. One place.

AO Smith Water Heater E3 Error — Sensor Wire Fault Fix

What Does This Error Mean?

E3 indicates a sensor wiring fault — the temperature sensor is reading a voltage that doesn't match any plausible water temperature, so the controller flags it as broken.
Usually the sensor wire has come loose at the controller, the sensor itself has failed, or moisture has gotten into the sensor connection.
This is a parts-level repair — replacement sensors are inexpensive but require a plumber to fit safely.

Affected Models

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

Common Causes

  • Sensor wire disconnected at the control board
  • Moisture in the sensor connector causing corrosion
  • Sensor probe failed (open or shorted)
  • Wire chewed by rodents (unconditioned spaces)
  • Control board sensor input damaged

How to Fix It

  1. Power the unit down.

    Cut the breaker for electric heaters.
    Turn the gas valve to off for gas heaters.
    You should always work with the heater de-energized — there's no urgent reason to leave it on while diagnosing E3.

  2. Don't open the access panel yourself.

    On AO Smith heaters, the temperature sensor is connected to the control board behind a sealed access panel.
    Opening it can void warranty and risks gas-side or electrical exposure.
    This is genuinely plumber-level work — they're trained on these specific units.

  3. Check the install for water damage.

    While waiting for service, walk around the heater.
    Is there any drip or leak?
    Water on the controller area can corrode sensor connections and is a common cause of E3.
    Document for the plumber so they know to inspect the controller area carefully.

  4. Check for unusual conditions.

    Has anyone moved the heater recently?
    Has there been construction nearby (vibration loosens connectors)?
    Has it been in storage and reinstalled?
    Note anything unusual for the technician — context speeds diagnosis.

  5. Call a licensed plumber.

    Have your model and serial number ready.
    Most plumbers can fix E3 in one visit if they have the sensor on hand.
    Replacement sensors are 20–50 USD plus an hour of labor.
    The fix is straightforward once on site.

Frequently Asked Questions

Will the heater still work with E3 showing?

Most AO Smith heaters lock out heating when E3 is active, falling back to a safe-mode that produces tepid water at best.
You won't get hot showers until E3 is fixed.
Some models cycle on and off unpredictably until the fault clears — don't rely on it for cooking or hygiene.

Can sediment cause E3?

Indirectly, yes.
Heavy sediment around the sensor probe can insulate it from accurate water temperature reading.
Eventually the sensor reads erratically and the controller flags E3.
An annual flush of the tank prevents this — the plumber will do it during the service call.