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Error code reference

A.O. Smith Water Heater Error Code 311

Output thermistor fault.

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

Output thermistor fault.

First step from the service manual

Check that wire connections are secure and free of burns or cuts; the output thermistor is located on the heat exchanger on the right side.

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

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  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
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What 311 means on a A.O. Smith water heater

Error code 311 on an A.O. Smith water heater indicates an output thermistor fault. The output thermistor is a temperature-sensing resistor located on the heat exchanger on the right side of the unit. This sensor continuously monitors the temperature of the water leaving the heat exchanger, providing real-time feedback to the control board so it can regulate burner output and maintain safe operating temperatures. When the control board detects a signal from this thermistor that falls outside the expected range, whether an open circuit, short circuit, or implausible temperature reading, it triggers code 311 and typically shuts down the heater to prevent overheating or damage.

The most common starting point for diagnosing this fault is a thorough inspection of the wiring harness connected to the output thermistor. Loose connectors, corroded terminals, pinched wires, or heat damage such as burns or cuts along the harness can all interrupt the signal between the thermistor and the control board, causing the fault without the thermistor itself being defective. Resistance across the thermistor leads should match the expected value for the current ambient or water temperature as specified for that sensor type.

If the wiring checks out and connections are secure and undamaged, the thermistor itself may have drifted out of specification or failed entirely. A failed thermistor will typically read either infinite resistance (open) or near-zero resistance (short), neither of which corresponds to a valid temperature. In some cases, scale buildup or debris around the sensor on the heat exchanger can affect readings. Control board failure is possible but is the least likely cause and should only be considered after the sensor and wiring have been ruled out.

Source: manufacturer service documentation.

Common causes of 311

  1. 01

    Loose or disconnected wiring at the thermistor

    Wiring / connection

    The connector at the output thermistor on the right side of the heat exchanger may have vibrated loose or was never fully seated. Inspect the connector and pins for secure engagement and push firmly to reseat if needed.

  2. 02

    Burned or damaged wiring harness

    Part replacement

    Heat exposure near the heat exchanger can burn or melt the insulation on the thermistor wires, causing an open or short circuit in the signal line. Visually trace the harness from the thermistor connector back toward the control board looking for any charring, cuts, or exposed conductors.

  3. 03

    Corroded or contaminated connector terminals

    Wiring / connection

    Moisture or condensation near the heat exchanger can cause oxidation on the thermistor connector pins, increasing resistance in the circuit and producing an out-of-range signal. Clean corroded terminals carefully and check if the fault clears.

  4. 04

    Failed output thermistor

    Part replacement

    The thermistor itself may have failed, presenting either an open circuit or a short circuit instead of a resistance value that corresponds to a valid temperature. If wiring and connections are intact, measure resistance across the thermistor leads and compare to the expected value for the sensor type.

  5. 05

    Defective control board

    Part replacement

    If the thermistor resistance measures correctly and all wiring is confirmed intact, the control board may be misreading the sensor signal due to an internal component failure. Control board replacement should only be considered after all other causes have been eliminated.

See the test procedure for each cause

Frequently asked questions about 311

What does 311 mean on a A.O. Smith water_heater?
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.
How do I fix error code 311 on my A.O. Smith water heater?
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.
Is error code 311 on an A.O. Smith water heater dangerous?
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.
Will resetting the water heater clear error code 311?
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.
How much does it cost to repair error code 311 on an A.O. Smith water heater?
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.

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