mypros+
Error code reference

A.O. Smith Water Heater Error Code 3

Ignition/flame failure

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

Ignition/flame failure - water heater has reached maximum number of retries and is locked out for one hour.

First step from the service manual

Ensure gas supply is turned on and supply/manifold gas pressures are within requirements.

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

MyPros+ Max

Get the full diagnostic procedure for 3.

  • 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
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What 3 means on a A.O. Smith water heater

Error code 3 on an A.O. Smith water heater indicates an ignition or flame failure lockout. The control board attempted to ignite the burner assembly multiple times, exhausted its maximum retry count, and has now locked out the ignition system for a one-hour period. During this lockout, the heater will not attempt another ignition cycle until the timer expires or the unit is manually reset. The ignition sequence involves the gas valve opening, the igniter generating a spark or hot surface ignition, and the flame sensor confirming that a stable flame is present. A failure at any point in this chain will register as a failed ignition attempt.

The most fundamental cause of this fault is insufficient gas supply. The control system requires supply gas pressure and manifold pressure to fall within specific operating ranges. If upstream shutoff valves are partially or fully closed, the gas meter is in an error state, or there is a pressure drop due to other appliances running simultaneously, the burner may receive inadequate fuel to sustain ignition. Verifying that the manual gas shutoff valve at the heater is fully open and that line pressure meets the required specification is the correct first diagnostic step.

Beyond gas supply issues, the fault can originate from a failed or contaminated flame sensor, a defective igniter, a faulty gas valve that is not opening correctly, or venting problems that cause flame instability or immediate extinguishment after ignition. Blocked flue passages or high wind backdrafting can also disrupt combustion enough to prevent a stable flame signal. After the one-hour lockout period clears, the unit will reattempt ignition, but the underlying cause must be corrected to prevent an immediate return of the fault.

Source: manufacturer service documentation.

Common causes of 3

  1. 01

    Gas supply shutoff or low pressure

    Wiring / connection

    The manual gas shutoff valve at the heater may be partially or fully closed, or line pressure may be outside the required range. Check that the valve is fully open and confirm supply and manifold pressures meet specifications.

  2. 02

    Failed or dirty flame sensor

    Part replacement

    The flame sensor rod can accumulate oxidation or residue over time, preventing it from generating a reliable signal even when a flame is present. The sensor should be inspected and cleaned or replaced if the ceramic insulator is cracked or the rod is heavily corroded.

  3. 03

    Defective igniter

    Part replacement

    A hot surface igniter or spark igniter that has cracked, weakened, or failed will not generate the energy needed to initiate combustion. Testing igniter output during the ignition sequence confirms whether it is operating correctly.

  4. 04

    Faulty gas valve

    Part replacement

    If the gas valve is not opening during an ignition attempt, no fuel reaches the burner regardless of igniter condition. A gas valve that sticks, fails electrically, or does not receive the correct signal from the control board will cause repeated ignition failures.

  5. 05

    Venting or combustion air obstruction

    Wiring / connection

    A blocked flue, debris in the vent termination, or inadequate combustion air supply can extinguish the flame immediately after ignition, triggering a flame failure event. Inspect the full vent path and air intake for blockages, nesting material, or ice buildup.

See the test procedure for each cause

Frequently asked questions about 3

What does 3 mean on a A.O. Smith water_heater?
Error code 3 means the water heater attempted to ignite the burner the maximum number of times allowed and failed on each attempt. As a result, the control board has locked out the ignition system for one hour before it will try again. The code points to a problem somewhere in the ignition or gas delivery system. Common culprits include a closed gas valve, low gas pressure, a failed igniter, or a faulty flame sensor.
Can I reset the error code 3 lockout myself, or do I have to wait an hour?
The lockout timer is built into the control board and is designed to run for one hour before the unit automatically reattempts ignition. Many A.O. Smith models allow a manual reset by pressing the reset button on the control, which can clear the lockout early. However, if the root cause has not been corrected, the heater will simply run through its retry sequence again and return to the same locked-out state.
Is it safe to continue using the water heater when error code 3 is displayed?
When error code 3 is active, the burner is not firing and the heater is not producing hot water, so there is no active combustion hazard during the lockout period itself. The lockout is a protective function, not a sign of immediate danger. However, you should not attempt to bypass the control system or force repeated ignition without diagnosing the cause, as a persistent gas supply issue or component failure should be properly addressed.
How much does it cost to repair an A.O. Smith water heater showing error code 3?
Repair costs depend entirely on the root cause. If the fix is simply opening a gas shutoff valve or clearing a vent obstruction, the cost is minimal. Replacing a flame sensor typically runs in the range of $50 to $150 in parts and labor. A gas valve replacement is more involved and can cost anywhere from $200 to $500 or more depending on the model and labor rates in your area.
Will the error code 3 clear on its own after the one-hour lockout?
The one-hour lockout will expire on its own, after which the control board will make another ignition attempt. If that attempt succeeds, the fault code will clear and normal operation will resume. If the underlying problem has not been fixed, the heater will fail to ignite again, accumulate retry failures, and return to the locked-out state with error code 3 displayed again.

Related A.O. Smith water heater codes

All A.O. Smith water heater codes
View all 36 A.O. Smith water heater codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass