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Error code reference

A.O. Smith Water Heater Error Code 4

Weak flame sensor current.

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

Weak flame sensor current.

First step from the service manual

Ensure flame sensor is making good contact with the burner flame and flame is steady.

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

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What 4 means on a A.O. Smith water heater

Error Code 4 on an A.O. Smith water heater indicates that the flame sensor is detecting a weak electrical current from the burner flame. The flame sensor operates through a process called flame rectification, where the burner flame acts as a conductor and allows a small DC microamp current to flow through the sensor rod. The control board monitors this current continuously during burner operation, and when the signal falls below an acceptable threshold, Code 4 is triggered to indicate the flame signal is too weak to be reliably confirmed.

The most common cause of a weak flame sensor signal is poor physical contact between the sensor rod and the burner flame itself. If the flame is not enveloping the sensor tip properly, or if the flame is unsteady due to gas pressure irregularities or combustion air issues, the rectified current will drop. A dirty or oxidized sensor rod is another frequent cause, as any coating on the rod surface will impede the electrical contact needed for proper flame rectification.

Additionally, wiring issues between the flame sensor and the control board can introduce resistance into the circuit, reducing the measured current even when the flame itself is adequate. The control board's ability to read the microamp signal accurately can also be compromised by a failing board. Diagnosing Code 4 starts with visually confirming the flame is steady and the sensor rod is properly positioned within the flame envelope, then inspecting and cleaning the sensor rod before moving to electrical and component checks.

Source: manufacturer service documentation.

Common causes of 4

  1. 01

    Flame sensor not properly contacting the flame

    Wiring / connection

    If the sensor rod tip is not positioned within the burner flame envelope, the rectified current will be too low. Visually inspect the sensor rod position relative to the active flame during burner operation.

  2. 02

    Dirty or oxidized flame sensor rod

    Wiring / connection

    Carbon buildup or oxidation on the sensor rod surface insulates it and reduces the microamp signal. The rod can be cleaned with fine steel wool or emery cloth to restore proper conductivity.

  3. 03

    Unsteady or weak burner flame

    Wiring / connection

    Low gas pressure, a partially clogged burner orifice, or combustion air problems can cause a flickering or undersized flame that intermittently loses contact with the sensor. Check gas supply pressure and burner condition.

  4. 04

    Damaged or corroded flame sensor wiring

    Wiring / connection

    Faulty wiring or a loose connector between the sensor rod and the control board adds resistance to the signal circuit, reducing the current the board reads. Inspect the wiring harness and connector for corrosion, fraying, or loose terminals.

  5. 05

    Failed flame sensor rod or control board

    Part replacement

    A cracked sensor rod or a control board that can no longer accurately measure the microamp signal will produce a persistent Code 4 even after cleaning and flame adjustments. Component replacement is required if all other causes are ruled out.

See the test procedure for each cause

Frequently asked questions about 4

What does 4 mean on a A.O. Smith water_heater?
Error Code 4 means the flame sensor is detecting a weaker-than-acceptable current from the burner flame. The water heater uses a flame rectification process to confirm the burner is lit, and when that signal is too low, the unit faults and shuts down. This is a safety feature to prevent unconfirmed or unsafe burner operation. The issue is typically related to the sensor rod condition, its position in the flame, or the quality of the burner flame itself.
How do I fix Error Code 4 on my A.O. Smith water heater?
Start by checking that the burner flame is steady and fully surrounding the tip of the flame sensor rod. If the flame looks normal, turn off power to the unit and clean the sensor rod with fine steel wool or emery cloth to remove any oxidation or carbon deposits. Also inspect the wiring connector at the sensor and control board for corrosion or looseness. If cleaning and connection checks do not resolve the error, the sensor rod or control board may need to be replaced.
Is Error Code 4 on an A.O. Smith water heater dangerous?
The error itself is a protective response, not a dangerous condition. The control board shuts the burner down because it cannot confidently confirm the flame is burning correctly. This prevents uncontrolled gas combustion. However, you should not repeatedly attempt to restart the unit without diagnosing the root cause, as persistent ignition attempts with an unresolved issue can create unsafe conditions.
Will resetting my A.O. Smith water heater clear Error Code 4?
A reset may temporarily clear the code and allow the unit to attempt ignition again, but if the underlying cause is not corrected, Code 4 will return. Resetting without addressing the flame sensor condition, flame quality, or wiring issue is not a long-term fix. Use a reset only as part of the diagnostic process after you have made a corrective adjustment, to confirm whether the issue has been resolved.
How much does it cost to repair an A.O. Smith water heater showing Error Code 4?
If the fix is simply cleaning the flame sensor rod, the cost is minimal and can often be done without professional help. If the flame sensor rod itself needs replacement, parts typically cost in the range of $15 to $50 depending on the model. A control board replacement, if required, can range from $100 to $300 or more for parts alone. Adding professional labor, a service call to diagnose and repair this error generally runs between $100 and $350 depending on your location and what component ultimately needs attention.

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