mypros+
Error code reference

A.O. Smith Water Heater Error Code 631

Pump failure.

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

Pump failure.

First step from the service manual

Check to see whether the pump connected to the PCB works properly.

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

MyPros+ Max

Get the full diagnostic procedure for 631.

  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What 631 means on a A.O. Smith water heater

Error code 631 on an A.O. Smith water heater indicates a pump failure condition. This code is generated when the control board, referred to as the PCB (printed circuit board), detects that the circulation or recirculation pump is not operating as expected. The PCB continuously monitors pump operation, and when it cannot confirm proper pump function, it triggers this fault to protect the system and alert the technician.

The pump in these systems is responsible for circulating water through the heat exchanger or recirculation loop. When the pump fails, water flow through the system is compromised, which can lead to overheating, inadequate hot water delivery, or damage to internal components. The PCB serves as the control hub that powers and monitors the pump, so any break in the electrical connection between the PCB and the pump, or any mechanical failure of the pump itself, will result in this error.

The first diagnostic step is to verify whether the pump connected to the PCB is functioning properly. This includes checking for power output at the pump terminals on the PCB, inspecting the wiring harness for damage or loose connections, and testing the pump motor directly. A pump that receives proper voltage but fails to run typically has a failed motor or impeller obstruction. If the PCB is not delivering the correct output signal or voltage to the pump, the control board itself may be at fault.

Source: manufacturer service documentation.

Common causes of 631

  1. 01

    Failed pump motor

    Part replacement

    The pump motor may have burned out or seized, preventing it from running even when power is supplied. If the pump receives voltage at its terminals but does not spin, the motor has likely failed and the pump will need replacement.

  2. 02

    Loose or damaged wiring between PCB and pump

    Wiring / connection

    A disconnected, corroded, or broken wire in the harness between the control board and the pump can interrupt the signal or power needed to run the pump. Inspect all connector pins and wire insulation for damage or corrosion.

  3. 03

    Pump impeller obstruction

    Wiring / connection

    Debris, scale buildup, or sediment can jam the pump impeller, causing the motor to stall or overheat and trip the fault. The pump may hum but fail to move water in this scenario.

  4. 04

    PCB not supplying correct output to pump

    Part replacement

    If the control board itself is faulty, it may fail to deliver the proper voltage or control signal to the pump terminal. If voltage at the PCB pump output is absent or incorrect while the pump wiring is intact, the board may need replacement.

  5. 05

    Pump capacitor failure

    Part replacement

    Some pump motors rely on a start or run capacitor to initiate and maintain rotation. A failed capacitor can prevent the pump from starting even though the motor windings are intact.

See the test procedure for each cause

Frequently asked questions about 631

What does 631 mean on a A.O. Smith water_heater?
Error code 631 on an A.O. Smith water heater indicates a pump failure. The control board has detected that the pump connected to the PCB is not operating properly. This could be due to a failed pump motor, a wiring issue, an obstruction, or a faulty control board output. The system flags this fault to prevent damage and alert the user that service is required.
Can I fix error code 631 myself?
Some basic checks, such as inspecting the wiring connections between the PCB and the pump, can be performed by a knowledgeable DIYer with the power safely shut off. However, testing pump voltage output, diagnosing PCB faults, and replacing components typically require electrical testing tools and familiarity with water heater systems. If you are not comfortable working with electrical components, a qualified technician should handle the repair.
Is it safe to continue using my water heater while error 631 is displayed?
It is not recommended to continue operating the water heater with a pump failure fault active. The pump plays a critical role in water circulation, and running the system without proper pump operation can lead to overheating or damage to internal components. Shut the unit down and have it inspected before resuming use.
How much does it cost to repair error code 631 on an A.O. Smith water heater?
Repair costs depend on the root cause. Pump replacement parts typically range from $50 to $250 depending on the model and pump type, with labor adding $75 to $200 or more. If the PCB is at fault, control board replacement can range from $150 to $400 for the part alone. A technician diagnosis visit may also carry its own service fee.
Will resetting the water heater clear error code 631?
Resetting the unit may temporarily clear the fault code, but if the underlying pump problem has not been resolved, the error will return once the control board performs its next pump check. A reset should only be attempted after the pump, wiring, and PCB connections have been inspected and any issues corrected. Do not rely on repeated resets as a substitute for proper diagnosis and repair.

Related A.O. Smith water heater codes

All A.O. Smith water heater codes
View all 36 A.O. Smith water heater codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass