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Error code reference

A.O. Smith Water Heater Error Code 551

Abnormal solenoid gas valves

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

Abnormal solenoid gas valves — driving circuit fault for the solenoid gas valves.

First step from the service manual

If hot water is shut off but flames are still visible through the burner sight window, immediately shut off gas and power and contact Technical Service Department.

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

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  • 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
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What 551 means on a A.O. Smith water heater

Error code 551 on an A.O. Smith water heater indicates a fault in the driving circuit for the solenoid gas valves. The solenoid gas valve is an electrically controlled component that opens and closes to regulate gas flow to the burner. The control board sends specific voltage signals to energize or de-energize the solenoid coil, which physically moves an internal plunger to allow or block gas flow. When the control board detects an abnormal condition in this driving circuit, such as incorrect voltage, an open circuit, a short circuit, or a stuck valve, it triggers error code 551 and shuts down the heating cycle as a safety measure.

The driving circuit includes the solenoid valve coil itself, the wiring harness connecting it to the control board, and the output drivers on the control board that supply current to the valve. A fault can originate at any point in this circuit. A failed solenoid coil will show abnormal resistance readings. Damaged or corroded wiring can cause open or short circuit conditions. A faulty control board output driver may fail to deliver the correct current even if the valve and wiring are intact.

The most critical safety concern with this fault is the possibility of a stuck-open solenoid valve, which can allow gas to flow to the burner even after a shutdown command is issued. If flames remain visible through the burner sight window after hot water demand has stopped, the valve has failed in the open position. This condition requires immediate gas shutoff and power disconnection before any further diagnostics or repair attempts.

Source: manufacturer service documentation.

Common causes of 551

  1. 01

    Failed solenoid gas valve coil

    Part replacement

    The coil inside the solenoid valve can burn out or develop an open or short circuit, causing abnormal resistance readings that the control board detects as a driving circuit fault. The valve assembly typically needs replacement when the coil fails.

  2. 02

    Stuck-open solenoid valve

    Part replacement

    The solenoid valve plunger can become mechanically stuck in the open position, allowing continuous gas flow even after the control board commands a shutoff. This is identified by visible burner flames persisting after the heating cycle should have ended.

  3. 03

    Damaged or corroded wiring harness

    Wiring / connection

    Wiring between the control board and the solenoid valve can develop opens, shorts, or high-resistance connections due to corrosion, heat damage, or physical wear. Inspecting the connector pins and wire insulation along the entire harness can identify this cause.

  4. 04

    Faulty control board output driver

    Part replacement

    The output driver circuit on the control board that supplies current to the solenoid valve can fail, resulting in incorrect voltage or no voltage being delivered even when the valve and wiring are intact. This is typically diagnosed after the valve and wiring have been confirmed good.

  5. 05

    Loose or corroded electrical connections

    Wiring / connection

    Poor connections at the solenoid valve plug or control board terminals can cause intermittent or abnormal circuit conditions that trigger error 551. Cleaning and reseating all connectors in the gas valve circuit is an appropriate early diagnostic step.

See the test procedure for each cause

Frequently asked questions about 551

What does 551 mean on a A.O. Smith water_heater?
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.
Is error code 551 on an A.O. Smith water heater dangerous?
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.
Will resetting the water heater clear error code 551?
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.
Can I fix error code 551 myself?
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.
How much does it cost to repair error code 551 on an A.O. Smith water heater?
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.

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