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Error code reference

A.O. Smith Water Heater Error Code 991

Abnormal combustion

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

Abnormal combustion — persistent combustion issue requiring possible manifold pressure adjustment.

First step from the service manual

Refer to error code 101 troubleshooting steps.

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
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What 991 means on a A.O. Smith water heater

Error code 991 on an A.O. Smith water heater indicates an abnormal combustion condition that has persisted beyond what the control board considers a recoverable fault. This code is closely related to error code 101 and shares its diagnostic pathway, but the 991 designation signals that the combustion problem is recurring or sustained rather than a one-time event. The core issue involves the burner assembly failing to establish or maintain proper combustion characteristics, which the control system monitors through the flame sensor and pressure-related feedback.

A critical factor in resolving 991 is manifold gas pressure. If the pressure at the gas valve outlet is outside the specified operating range, the burner may light but burn erratically, incompletely, or fail to sustain a stable flame. This creates the abnormal combustion signature that triggers the fault. Manifold pressure adjustment requires a calibrated manometer and knowledge of the appliance's rated pressure specifications, making it a task best suited for a licensed technician.

Additional contributors include restricted venting that disrupts the air-to-fuel ratio, a fouled or failing flame sensor that misreports combustion status, a degraded gas valve that cannot regulate pressure accurately, or blockages in the burner ports themselves. Because this code specifically calls out the need for possible manifold pressure adjustment, that measurement should be a priority step after completing the standard 101 troubleshooting sequence.

Source: manufacturer service documentation.

Common causes of 991

  1. 01

    Incorrect manifold gas pressure

    Wiring / connection

    If the gas pressure at the valve outlet is too high or too low, the burner cannot sustain normal combustion. This must be measured with a manometer and adjusted at the gas valve.

  2. 02

    Restricted or blocked venting

    Wiring / connection

    A partially blocked flue or exhaust vent reduces available combustion air, causing incomplete or unstable burning. Inspect the full vent path for obstructions, debris, or bird nests.

  3. 03

    Faulty or contaminated flame sensor

    Part replacement

    A flame sensor coated with oxidation or carbon deposits may misread the burner's flame signal and report abnormal combustion even when the flame appears normal. Cleaning or replacing the sensor is a standard step in the 101 diagnostic sequence.

  4. 04

    Degraded or failing gas valve

    Part replacement

    A gas valve that cannot accurately regulate outlet pressure will produce inconsistent combustion. If manifold pressure cannot be properly set or drifts after adjustment, the valve itself may need replacement.

  5. 05

    Blocked or fouled burner ports

    Wiring / connection

    Debris, rust, or mineral deposits clogging the burner orifices disrupt the gas flow pattern and cause uneven or abnormal flame characteristics. Visual inspection of the burner assembly can identify this condition.

See the test procedure for each cause

Frequently asked questions about 991

What does 991 mean on a A.O. Smith water_heater?
Error code 991 indicates a persistent abnormal combustion condition in the burner system. The heater has detected that combustion is not occurring within normal parameters and has faulted out as a safety measure. This code specifically notes that manifold gas pressure may need adjustment as part of the resolution. Diagnostics should follow the same steps outlined for error code 101.
Will resetting the water heater clear error code 991?
A manual reset may temporarily clear the fault and allow the heater to attempt operation again, but if the underlying combustion problem is not corrected, the 991 code will return. Resetting without diagnosing and fixing the root cause, such as incorrect manifold pressure or a restricted vent, is not a permanent solution. The fault is persistent by nature, meaning the control board has already attempted to recover on its own before displaying 991.
Is it safe to continue using the water heater when error code 991 is displayed?
No. When the 991 code is active, the heater has shut down the burner as a precaution against abnormal combustion. Attempting to force operation without correcting the fault could pose a risk of incomplete combustion or carbon monoxide production. Leave the unit off and contact a licensed technician to inspect and repair the system before resuming normal use.
How do I fix error code 991 on an A.O. Smith water heater?
Begin by following the diagnostic steps for error code 101, as the 991 code shares that troubleshooting pathway. Check for venting obstructions, inspect the flame sensor, and verify the gas supply is adequate. If those steps do not resolve the issue, have a licensed technician measure the manifold gas pressure with a manometer and adjust it as needed at the gas valve. Component replacement such as a new gas valve or flame sensor may also be required depending on what the diagnostics reveal.
How much does it cost to repair a 991 error on an A.O. Smith water heater?
Repair costs vary depending on the root cause. A service call to diagnose and adjust manifold pressure typically ranges from $100 to $250. If a flame sensor replacement is needed, parts and labor generally run $150 to $350. A gas valve replacement, which is a more involved repair, can cost anywhere from $300 to $600 or more depending on the model and local labor rates. Getting a written estimate after diagnosis is the best approach before authorizing repairs.

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