A.O. Smith Water Heater Error Code 551

Error code 551 indicates an abnormal condition in the driving circuit for the solenoid gas valves. The control board has detected a fault, which could be a failed valve coil, a short or open circuit in the wiring, or a problem with the board's output driver. The heater shuts down as a safety response to this fault.

A reset may temporarily clear the fault display, but it will not correct an underlying hardware failure in the solenoid valve, wiring, or control board. If the root cause is not repaired, the error will return. Given the safety implications of this code, resetting without diagnosis is not recommended.

Repair costs depend on the failed component. A solenoid gas valve replacement typically ranges from $150 to $400 including parts and labor. Control board replacement can range from $200 to $500 or more depending on the model. A wiring harness repair is generally on the lower end of the cost range. A technician diagnosis fee may apply in addition to parts and labor costs.

This code can indicate a serious safety hazard if the solenoid valve is stuck in the open position. If you observe flames in the burner sight window after the heater has shut down or after hot water demand has ended, turn off the gas supply and disconnect power to the unit immediately. Do not attempt any repairs until gas flow has been confirmed stopped.

Some preliminary checks, such as inspecting wiring connections and looking for obvious damage, are within reach for experienced DIYers. However, because this fault involves the gas valve and a potential stuck-open valve condition, most repairs should be performed by a qualified technician. Working on gas valve components carries serious safety risks if gas flow is not properly controlled first.