A.O. Smith Water Heater Error Code 321

Code 321 indicates an inlet thermistor fault, meaning the control board is not receiving a valid signal from the temperature sensor located at the water inlet connection. This sensor measures incoming cold water temperature to help the unit regulate heating output. The fault can be caused by loose wiring, damaged wires, a corroded connector, or a failed thermistor. The unit will typically limit or shut down operation until the fault is resolved.

You should not continue operating the water heater while code 321 is active. Without a functioning inlet thermistor, the control board cannot accurately calculate heating requirements, which can lead to improper operation. The unit may shut itself down as a safety precaution, but it is best to address the fault promptly rather than attempt to override it.

Repair costs vary depending on the cause. If only a wiring connection needs to be reseated or a terminal cleaned, the cost may be limited to a service call fee, typically in the range of $75 to $150. Replacing the inlet thermistor itself generally adds $20 to $80 in parts. If the control board requires replacement, total repair costs can range from $200 to $500 or more depending on the model and labor rates in your area.

A homeowner comfortable with basic appliance inspection can start by checking that all wiring connections at the water inlet are secure and free of visible damage. If the wiring looks intact and the error persists, measuring thermistor resistance requires a multimeter and some familiarity with reading component specs. Replacing the inlet thermistor itself is a moderate difficulty repair, but if the control board is suspected, professional service is recommended.

A reset may temporarily clear the code, but it will return if the underlying cause has not been repaired. If the thermistor wiring is loose or the sensor itself has failed, the control board will detect the same out-of-range condition as soon as it runs its diagnostic checks again. Resetting without repairing the root cause is not a lasting solution.