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Error code reference

A.O. Smith Water Heater Error Code 13

Temperature sensor (thermistor) inside the immersion probe is open.

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

Temperature sensor (thermistor) inside the immersion probe is open.

First step from the service manual

Turn power off for 10 to 20 seconds then on again to clear the error code.

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

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What 13 means on a A.O. Smith water heater

Error code 13 on an A.O. Smith water heater indicates that the temperature sensor (thermistor) inside the immersion probe has developed an open circuit condition. The thermistor is a resistive temperature-sensing element embedded within the immersion probe assembly, which is inserted directly into the water heater's tank or heat exchanger to monitor water temperature in real time. An open circuit means the electrical path through the thermistor has been completely broken, preventing the control board from receiving any resistance-based temperature signal. Without this data, the unit cannot regulate heating safely and will shut down to prevent overheating or other thermal damage.

The most common causes of an open thermistor include physical damage to the sensor element, corrosion at the probe connector or wiring terminals, a broken wire in the sensor harness, or outright thermistor failure due to age or thermal stress. Because the probe is immersed in water, it is also susceptible to mineral scale buildup and corrosion that can compromise the sensor's integrity over time. In some cases, a loose or poorly seated connector between the probe and the control board can mimic an open circuit without the sensor itself being faulty.

The first diagnostic step is to cycle power off for 10 to 20 seconds, then restore it. This clears stored fault data and allows the control board to re-evaluate the sensor signal. If the error returns immediately after the reset, the fault is persistent and requires physical inspection of the immersion probe, its wiring harness, and all associated connectors. A technician can use a multimeter to check continuity and resistance across the thermistor leads to confirm whether the sensor is genuinely open or whether the fault lies in the wiring.

Source: manufacturer service documentation.

Common causes of 13

  1. 01

    Failed thermistor inside the immersion probe

    Part replacement

    The thermistor element itself has burned out or broken internally, creating an open circuit. This is the most frequent cause of a persistent code 13 and is confirmed by measuring infinite resistance across the sensor leads with a multimeter.

  2. 02

    Loose or corroded connector at the probe

    Wiring / connection

    The connector joining the immersion probe to the wiring harness can loosen over time or develop corrosion, interrupting the signal path. Inspecting and reseating or cleaning the connector terminals may resolve the fault without replacing the probe.

  3. 03

    Damaged wiring in the sensor harness

    Part replacement

    A broken, pinched, or chafed wire between the immersion probe and the control board can create an open circuit that mimics a failed thermistor. Visual inspection of the harness routing and a continuity check on each wire will identify this condition.

  4. 04

    Corrosion or scale buildup on the probe

    Part replacement

    Mineral deposits or corrosion on the immersion probe can degrade the sensor element over time, eventually causing it to fail open. Units in areas with hard water or high mineral content are particularly susceptible to this type of damage.

  5. 05

    Transient electrical fault or power surge

    Wiring / connection

    A brief power interruption, voltage spike, or electrical transient can cause the control board to log a false open-circuit reading. This is the least likely persistent cause but is addressed by the initial power-cycle reset procedure.

See the test procedure for each cause

Frequently asked questions about 13

What does 13 mean on a A.O. Smith water_heater?
Error code 13 means the temperature sensor (thermistor) inside the immersion probe is registering as an open circuit. The control board cannot receive a valid temperature reading, so the unit shuts down to prevent unsafe operation. This fault points to a failed thermistor, a wiring issue, or a faulty connection between the probe and the control board.
Will resetting the water heater clear error code 13?
Turning the power off for 10 to 20 seconds and then back on will clear the stored fault code and allow the unit to recheck the sensor. If the error code does not return after the reset, the fault was likely a transient electrical issue. If code 13 comes back immediately or shortly after the reset, the underlying problem with the thermistor or its wiring is still present and requires further diagnosis.
How do I fix error code 13 on an A.O. Smith water heater?
Start by cycling the power off for 10 to 20 seconds to clear the fault. If the code returns, inspect the immersion probe connector and wiring harness for loose connections, corrosion, or visible damage. Use a multimeter to check resistance across the thermistor leads: an open reading (infinite resistance) confirms the probe needs replacement. If wiring and connectors check out, replacing the immersion probe assembly is the appropriate repair.
Is it safe to use my water heater when error code 13 is displayed?
When code 13 is active, the water heater will typically shut down and not heat water, which is a protective response to the loss of temperature monitoring. Operating a water heater without a functioning temperature sensor creates a risk of uncontrolled heating or overheating. It is best to keep the unit off until the fault is diagnosed and repaired.
How much does it cost to repair error code 13 on an A.O. Smith water heater?
Repair costs depend on the root cause. If only a connector needs cleaning or reseating, the labor cost may be minimal, typically in the range of $50 to $150 for a service call. If the immersion probe assembly requires replacement, parts and labor combined generally fall in the range of $100 to $350, depending on the specific model and local labor rates. A full wiring harness replacement would be at the higher end of that range.

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