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Error code reference

A.O. Smith Water Heater Error Code 111

Ignition failure

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

Ignition failure — heater does not fire after three failed attempts at ignition.

First step from the service manual

Verify that the hi-limit switch is not tripped; turn power off, push the white button in to reset, then test the heater.

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

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What 111 means on a A.O. Smith water heater

Error code 111 on an A.O. Smith water heater indicates an ignition failure condition. Specifically, the control board has made three consecutive attempts to ignite the burner and has detected no successful flame establishment on any of those attempts. After the third failed attempt, the unit locks out and displays this code to prevent unburned gas from accumulating in the combustion chamber. The ignition sequence involves the control board energizing the igniter, opening the gas valve, and confirming flame presence through the flame sensor. A failure at any point in this sequence contributes to the lockout.

The most common starting point for diagnosing this error is the hi-limit switch, a thermal safety device that cuts power to the ignition system when water or component temperatures exceed a safe threshold. If the hi-limit has tripped, the igniter will not receive power and no ignition attempt can succeed. To reset it, turn off power to the unit, locate the white reset button on the hi-limit switch, and press it firmly until you feel or hear it click. Restore power and attempt to fire the heater again.

Beyond the hi-limit switch, code 111 can stem from gas supply problems, a fouled or failed flame sensor, a defective igniter, or a faulty gas valve. The control board must detect a valid flame signal within a set window after the gas valve opens. If the igniter is too weak to light the gas, or if the flame sensor cannot read the flame due to oxidation or carbon buildup, the board registers a failed attempt. Systematic component testing is required to isolate the root cause when a hi-limit reset does not resolve the lockout.

Source: manufacturer service documentation.

Common causes of 111

  1. 01

    Tripped hi-limit switch

    Wiring / connection

    The hi-limit switch is a thermal safety device that shuts down the ignition system when temperatures exceed safe limits. If it has tripped, resetting it by pressing the white button with power off is the required first step.

  2. 02

    Insufficient or interrupted gas supply

    Wiring / connection

    If the gas shutoff valve is partially closed, the supply pressure is low, or other gas appliances are drawing heavily from the same line, the burner may not receive enough fuel to sustain ignition. Check that the gas shutoff valve is fully open and that gas is available at the meter.

  3. 03

    Fouled or failed flame sensor

    Part replacement

    The flame sensor rod can develop an oxidized coating over time, reducing its ability to detect a flame and causing the control board to log a failed ignition attempt. The sensor may need to be cleaned or replaced.

  4. 04

    Weak or failed igniter

    Part replacement

    If the igniter is not producing a strong enough spark or has failed entirely, the gas will not light during the ignition window. Testing igniter output and inspecting for cracks or wear will confirm this condition.

  5. 05

    Defective gas valve

    Part replacement

    A gas valve that fails to open on command during the ignition sequence will prevent any fuel from reaching the burner. This is less common but should be tested if the igniter and flame sensor both check out correctly.

See the test procedure for each cause

Frequently asked questions about 111

What does 111 mean on a A.O. Smith water_heater?
Code 111 means the water heater attempted to ignite the burner three times without success and has locked out to prevent unburned gas accumulation. The control board monitors each ignition attempt and trips this lockout when no flame is confirmed after three tries. The error can result from a tripped hi-limit switch, a gas supply issue, a faulty igniter, a dirty flame sensor, or a defective gas valve. Diagnosing the hi-limit switch first is the recommended starting point.
How do I fix error code 111 on my A.O. Smith water heater?
Start by turning off power to the unit, locating the hi-limit switch, and pressing the white reset button firmly until it clicks. Restore power and test whether the heater fires successfully. If the error returns, check that the gas supply valve is fully open and that gas pressure is adequate. If the unit still fails to ignite, further testing of the flame sensor, igniter, and gas valve will be needed to identify the failed component.
Is error code 111 on an A.O. Smith water heater dangerous?
The lockout triggered by code 111 is itself a safety feature, designed to stop the ignition system before unburned gas can build up to a hazardous level. While the locked-out state is safe, you should not attempt to manually override the lockout or repeatedly reset the unit without identifying the underlying cause. If you smell gas at any point, leave the area, avoid operating any switches, and contact your gas utility before proceeding.
Will resetting the water heater clear error code 111?
Resetting the hi-limit switch may clear the code if a temperature overload was the cause, allowing the heater to resume normal operation. However, if an igniter, flame sensor, or gas valve problem is causing the repeated ignition failures, the code will return after another three failed attempts. A reset should be treated as a diagnostic step, not a permanent fix, unless the root cause has been confirmed and corrected.
How much does it cost to repair error code 111 on an A.O. Smith water heater?
Repair costs depend on the faulty component. If only a hi-limit switch reset is needed, there is no parts cost. Replacing a flame sensor or igniter typically falls in the range of $50 to $200 for parts and labor combined, depending on the technician's rates in your area. A gas valve replacement is a more involved repair and can range from $200 to $500 or more. Getting a diagnosis before authorizing parts replacement helps avoid unnecessary expense.

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