A.O. Smith Water Heater Error Code 311

Code 311 indicates an output thermistor fault. The output thermistor is mounted on the heat exchanger on the right side of the unit and monitors the temperature of water leaving the heat exchanger. The control board has detected that the signal from this sensor is outside the acceptable range, so the heater has shut down to protect itself. The cause could be a wiring issue or a failed sensor.

The heater shuts down when this fault is detected, which is a built-in safety response to an unreliable temperature reading. As long as the unit remains off and is not producing hot water, there is no immediate safety hazard in the home. Do not attempt to bypass the fault or force the heater to run while the sensor fault is unresolved, as the control board would lose the ability to accurately monitor output temperature.

If only a wiring repair or connector cleaning is needed, costs are minimal and may only involve a service call fee, typically in the range of $75 to $150 depending on your area. Replacing the output thermistor itself is generally an affordable repair, with parts and labor often falling in the $100 to $250 range. If the control board is ultimately at fault, that repair can range from $250 to $500 or more depending on the model and labor rates.

Start by inspecting the wiring connected to the output thermistor on the right side of the heat exchanger. Look for loose connectors, burned insulation, cuts, or corrosion on the terminals and reseat or repair any damaged connections. If the wiring is in good condition, measure the resistance of the thermistor itself to determine whether it has failed and needs to be replaced. If both the wiring and thermistor check out, the control board may require further evaluation.

A manual reset may temporarily clear the fault and allow the heater to restart, but if the underlying issue with the thermistor or its wiring has not been corrected, the code will return, often quickly. Resetting without diagnosing and repairing the root cause is not a long-term solution. Diagnose the wiring and sensor before relying on a reset.