A.O. Smith Water Heater Error Code 661

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.

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.

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.

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.

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.