A.O. Smith Water Heater Error Code 331

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.

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.

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.

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.

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.