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Error code reference

A.O. Smith Water Heater Error Code 661

Bypass valve fault.

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

Bypass valve fault.

First step from the service manual

Turn off power supply and check that all wire connections are secure and free of burns.

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

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  • Wiring diagram references and connector pinouts from the OEM service manual
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What 661 means on a A.O. Smith water heater

Error code 661 on an A.O. Smith water heater indicates a bypass valve fault. The bypass valve is a critical component in systems that use a mixing or recirculation loop, responsible for directing water flow between the heat exchanger and the distribution system. When the control board detects that the bypass valve is not operating within expected parameters, such as failing to open, close, or respond to control signals, it logs this fault code and may shut down or limit heater operation to prevent damage.

The most common root causes of a 661 fault fall into two categories: electrical connection failures and mechanical valve failures. Loose, corroded, or burned wiring connections at the bypass valve harness can interrupt the control signal, causing the board to read the valve as unresponsive. The first diagnostic step is to disconnect power and thoroughly inspect all wiring connections associated with the bypass valve for signs of looseness, corrosion, or heat damage.

If wiring checks out, the bypass valve actuator or solenoid itself may have failed internally, either through mechanical seizure or electrical open/short. In some cases, debris or scale buildup inside the valve body can prevent proper movement, particularly in areas with hard water. Systematic testing should proceed from the wiring harness through the actuator coil resistance and finally to the valve body itself to isolate the failed component before ordering replacement parts.

Source: manufacturer service documentation.

Common causes of 661

  1. 01

    Loose or disconnected wiring

    Wiring / connection

    Wiring connections at the bypass valve harness can loosen over time due to vibration or thermal cycling. Inspect all connectors at the valve and control board for secure seating and signs of looseness.

  2. 02

    Burned or corroded wire connections

    Part replacement

    Heat and moisture can cause terminal pins and wire insulation to corrode or burn, breaking the control circuit. Look for discoloration, melting, or white oxidation at connectors during your initial inspection.

  3. 03

    Failed bypass valve actuator or solenoid

    Part replacement

    The actuator or solenoid coil that drives the bypass valve can fail electrically, resulting in an open or short circuit that the control board detects as a fault. Testing coil resistance can confirm this failure.

  4. 04

    Mechanical seizure of the valve body

    Part replacement

    Scale buildup or debris, especially in hard water areas, can prevent the bypass valve from moving freely. The valve may be stuck fully open or fully closed even if the electrical components are functioning.

  5. 05

    Control board fault

    Part replacement

    If the valve and all wiring test correctly, the control board itself may be misreading the valve signal or failing to send the proper control output. This is the least common cause and should be diagnosed only after all other components are ruled out.

See the test procedure for each cause

Frequently asked questions about 661

What does 661 mean on a A.O. Smith water_heater?
Error code 661 indicates a bypass valve fault. The control board has detected that the bypass valve is not responding or operating as expected, which can disrupt water flow through the heating system. The heater may limit or stop operation until the fault is resolved. The first step is to turn off power and check all wiring connections at the bypass valve for security and signs of damage.
Can I fix error code 661 myself on my A.O. Smith water heater?
A homeowner comfortable with basic electrical inspection can start by turning off power and checking that all wire connections to the bypass valve are secure and free of burns or corrosion. However, if wiring looks intact and the fault persists, diagnosing a failed valve actuator or solenoid requires electrical testing tools and some technical knowledge. Component replacement is best handled by a qualified technician.
Is it safe to continue using my water heater when code 661 is showing?
It is not recommended to continue using the water heater while error 661 is active. The bypass valve fault means water flow through the system may not be properly controlled, which can lead to overheating or other damage. Power off the unit and have the fault diagnosed before resuming normal use.
Will resetting the water heater clear error code 661?
A simple reset may temporarily clear the display, but the fault code will return if the underlying problem with the bypass valve or its wiring has not been corrected. Resetting without repairing the cause does not fix the issue and could allow conditions that cause further damage to persist. Always address the root cause before relying on a reset.
How much does it cost to repair error code 661 on an A.O. Smith water heater?
Repair costs depend on which component has failed. If the fix is simply reseating a loose wiring connector, costs may be limited to a service call fee, typically in the range of $75 to $150. If the bypass valve actuator or valve body requires replacement, parts and labor combined can range from $150 to $400 or more depending on the specific model and local labor rates.

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