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Error code reference

A.O. Smith Water Heater Error Code 391

Air-fuel ratio rod failure.

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

Air-fuel ratio rod failure.

First step from the service manual

Check that wire connections are secure and free of burns or cuts.

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

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What 391 means on a A.O. Smith water heater

Error code 391 on an A.O. Smith water heater indicates a failure in the air-fuel ratio (AFR) rod circuit. The AFR rod is a sensing component used in the combustion system to monitor the air-to-fuel mixture during burner operation. It works by detecting ionization in the burner flame, which allows the control board to verify that combustion is occurring at the correct ratio. When the control board cannot establish or maintain a proper signal from the AFR rod, it triggers this fault code and shuts down the burner as a safety measure.

The most common starting point for diagnosing error 391 is a thorough inspection of the wiring connections leading to the AFR rod. Loose, corroded, burned, or physically damaged wires can interrupt the low-level electrical signal that the rod depends on to communicate with the control board. Heat exposure over time can degrade wire insulation and connector integrity, making this a frequent failure point even when the rod itself is still functional.

If the wiring checks out as intact and secure, the AFR rod itself may be contaminated, cracked, or failed. Carbon buildup or oxidation on the rod tip can prevent proper ionization sensing. The control board is also a possible failure point if it cannot correctly interpret or supply the signal circuit, though this is typically diagnosed after ruling out the rod and wiring.

Source: manufacturer service documentation.

Common causes of 391

  1. 01

    Loose or damaged wiring connections

    Wiring / connection

    Wires leading to the AFR rod may have come loose or suffered burns, cuts, or corrosion over time. Visually inspect all connectors and wire insulation along the AFR rod circuit for any visible damage.

  2. 02

    Contaminated or fouled AFR rod

    Part replacement

    Carbon deposits or oxidation on the tip of the AFR rod can prevent it from sensing the ionization signal in the flame. The rod tip should be inspected for buildup and cleaned or replaced if fouled.

  3. 03

    Failed AFR rod

    Part replacement

    The rod itself may be cracked or internally failed, producing no signal even when wiring and combustion conditions are correct. Replacing the rod is required in this case.

  4. 04

    Faulty control board

    Part replacement

    If the wiring and AFR rod both test as functional, the control board may be unable to properly process the AFR rod signal. This is typically the last component diagnosed after other causes are ruled out.

See the test procedure for each cause

Frequently asked questions about 391

What does 391 mean on a A.O. Smith water_heater?
Error code 391 indicates an air-fuel ratio rod failure. The AFR rod is a combustion sensing component that monitors the burner flame, and this code means the control board is not receiving a valid signal from it. The heater shuts down the burner when this fault is detected to prevent unsafe operation. Diagnosis starts with checking the wiring connections to the rod for burns, cuts, or looseness.
Can I fix error code 391 myself?
A knowledgeable DIYer can start by inspecting the wiring connections to the AFR rod for visible damage or loose connectors, which is the first recommended diagnostic step. If the wiring appears intact, replacing the AFR rod itself is a more involved repair that requires comfort working around gas combustion components. Any repairs involving the gas valve, control board, or internal combustion system are best left to a licensed technician.
Is error code 391 on an A.O. Smith water heater dangerous?
The heater is designed to shut down the burner when error 391 is detected, which reduces the immediate risk of unsafe combustion. However, you should not attempt to bypass the fault or force the unit to operate while this code is active. Leave the unit off and have it diagnosed before restoring normal operation.
Will resetting the water heater clear error code 391?
A manual reset may temporarily clear the fault and allow the unit to attempt ignition again, but if the underlying problem with the AFR rod or its wiring has not been resolved, the error will return. Resetting without addressing the root cause is not a fix and may result in repeated shutdowns. The fault should be properly diagnosed and repaired before relying on the unit.
How much does it cost to repair error code 391 on an A.O. Smith water heater?
Repair costs vary depending on which component has failed. If only a wiring repair or connector replacement is needed, costs tend to be on the lower end, typically in the range of $75 to $200 including labor. Replacing the AFR rod itself may cost between $150 and $350 depending on parts and labor rates in your area. If the control board requires replacement, costs can range from $300 to $600 or more.

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