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Error code reference

A.O. Smith Water Heater Error Code 331

Mixing thermistor fault.

A.O. Smith codesA.O. Smith water heater codes

Mixing thermistor fault.

First step from the service manual

Check that wire connections are secure and free of burns or cuts; the mixing thermistor is embedded in the water outlet connection.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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  • Wiring diagram references and connector pinouts from the OEM service manual
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What 331 means on a A.O. Smith water heater

Error code 331 on an A.O. Smith water heater indicates a mixing thermistor fault. The mixing thermistor is a temperature-sensing resistor embedded directly in the water outlet connection of the unit. Its job is to monitor the temperature of the water leaving the heater, providing real-time feedback to the control board so the system can regulate mixing valve operation and maintain safe, consistent outlet water temperatures. When the control board detects a signal from this thermistor that falls outside the expected resistance or voltage range, it triggers error 331 to alert the technician that the sensor circuit is compromised.

The most common causes of this fault are wiring issues at the thermistor connector. Loose, burned, or cut wires between the thermistor and the control board can produce erratic or absent signals that the board interprets as a sensor failure. Because the thermistor is embedded in the outlet connection, it is also exposed to heat cycling and potential moisture, which can degrade the connector over time. In some cases, the thermistor itself has failed internally, shifting its resistance output outside the acceptable range for the control board.

Diagnosis should begin with a close visual inspection of all wire connections leading to the mixing thermistor, checking specifically for signs of burning, corrosion, loose pins, or physical damage to the wire insulation. If the wiring checks out, the thermistor resistance should be measured and compared against the expected values for the current water temperature. A reading that is open-circuit, short-circuit, or far outside the normal curve confirms thermistor failure and replacement of the component or outlet assembly will be necessary.

Source: manufacturer service documentation.

Common causes of 331

  1. 01

    Loose or disconnected wiring at the thermistor

    Wiring / connection

    The connector at the mixing thermistor can work loose over time due to vibration or heat cycling. Check that the connector is fully seated and that no pins have backed out of the housing.

  2. 02

    Burned or damaged wiring

    Part replacement

    Wires running to the mixing thermistor can be damaged by contact with hot surfaces or pinching during service. Inspect the full wire harness for visible burn marks, melted insulation, or cuts.

  3. 03

    Failed mixing thermistor

    Part replacement

    The thermistor itself can fail internally due to age, thermal stress, or moisture intrusion, causing its resistance output to shift outside the acceptable range. A multimeter reading of open-circuit or short-circuit on the thermistor terminals confirms failure.

  4. 04

    Corroded connector terminals

    Wiring / connection

    Because the thermistor is embedded in the water outlet connection, moisture exposure can corrode the electrical terminals over time, increasing resistance in the circuit and producing a fault signal.

  5. 05

    Faulty control board

    Part replacement

    If the wiring and thermistor both test within spec, the control board's sensor input circuit may have failed, causing it to misread a valid thermistor signal as a fault. This is the least common cause and should only be considered after all other components are cleared.

See the test procedure for each cause

Frequently asked questions about 331

What does 331 mean on a A.O. Smith water_heater?
Error 331 indicates a mixing thermistor fault. The mixing thermistor is a temperature sensor embedded in the water outlet connection, and the control board has detected that the sensor's signal is outside the expected range. This is typically caused by a loose wire connection, damaged wiring, or a failed thermistor. The heater may shut down or limit operation until the fault is corrected.
How do I fix error 331 on my A.O. Smith water heater?
Start by inspecting all wire connections at the mixing thermistor located at the water outlet connection, looking for loose connectors, burns, or cuts. If the wiring looks intact, use a multimeter to test the thermistor's resistance and check whether the reading is consistent with the current water temperature. A reading that shows open-circuit or short-circuit means the thermistor needs to be replaced. If both the wiring and thermistor test normal, the control board may need evaluation.
Is error 331 on an A.O. Smith water heater a safety issue?
Yes, the mixing thermistor plays a role in monitoring outlet water temperature, so a fault with this sensor means the unit cannot accurately verify that water is being delivered at a safe temperature. The control board may restrict or shut down operation as a precaution. You should resolve this fault before relying on the heater for normal use.
Will resetting my A.O. Smith water heater clear error 331?
A reset may temporarily clear the code, but if the underlying wiring or thermistor problem is still present, the fault will return. Error 331 reflects an active hardware issue in the sensor circuit, so a reset alone is not a fix. The thermistor connections and the sensor itself need to be inspected and tested to resolve the fault permanently.
How much does it cost to repair error 331 on an A.O. Smith water heater?
If the fix is simply re-seating a loose connector, there is no parts cost and only minimal labor time involved. Replacing the mixing thermistor or the associated wiring harness typically costs between $50 and $200 in parts, depending on the specific model and component. If the control board is found to be the cause, board replacement can range from $150 to $400 or more. Professional labor charges will add to these estimates.

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