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Error code reference

A.O. Smith Water Heater Error Code 10

Air pressure switch contacts remain open longer than 5 seconds after the combustion fan is energized (LDO-SW).

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

Air pressure switch contacts remain open longer than 5 seconds after the combustion fan is energized (LDO-SW).

First step from the service manual

Clean the screen on the combustion air intake chamber, retest.

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

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What 10 means on a A.O. Smith water heater

Error code 10 on an A.O. Smith water heater indicates that the air pressure switch (labeled LDO-SW) contacts have remained in the open position for more than 5 seconds after the combustion fan has been energized. Under normal operation, the combustion fan creates a negative pressure differential in the air intake chamber, which causes the pressure switch contacts to close and signal the control board that adequate airflow is present. When that closure does not occur within the 5-second window, the control board registers this fault and locks out the ignition sequence as a safety measure to prevent unvented combustion.

The air pressure switch is a critical safety component in induced-draft and power-vent water heaters. It monitors the pressure differential created by the combustion blower to confirm that combustion air is flowing properly before allowing the gas valve to open. If the switch contacts fail to close, the heater interprets this as a potential blocked flue, failed blower, or obstructed intake, any of which could result in incomplete combustion or dangerous exhaust spillage inside the home.

The most common trigger for this fault is a blocked or restricted combustion air intake screen. Lint, debris, insects, and dust can accumulate on the intake screen and reduce airflow enough that the blower cannot generate sufficient negative pressure to actuate the switch. Clearing the intake screen is always the recommended first diagnostic step. If cleaning the screen does not resolve the fault, further diagnosis should focus on the blower motor, the pressure switch itself, and the tubing connecting the switch to the air chamber.

Source: manufacturer service documentation.

Common causes of 10

  1. 01

    Blocked or dirty combustion air intake screen

    Wiring / connection

    Lint, dust, insects, and debris can accumulate on the intake screen and restrict airflow enough to prevent the blower from generating the pressure differential needed to close the switch. Inspect and clean the screen as the first step.

  2. 02

    Combustion blower motor failure or weak performance

    Part replacement

    If the blower motor is running slowly or intermittently due to worn bearings or electrical issues, it may not produce enough airflow to actuate the pressure switch within the 5-second window. Listen for unusual noise and verify the blower spins freely.

  3. 03

    Faulty air pressure switch (LDO-SW)

    Part replacement

    The pressure switch itself can fail mechanically or electrically, causing the contacts to remain open even when proper pressure is present. This can be tested by checking continuity across the switch contacts when the blower is running.

  4. 04

    Blocked or disconnected pressure switch tubing

    Wiring / connection

    The small rubber or plastic tubing that connects the air pressure switch to the blower housing can become cracked, kinked, disconnected, or clogged with condensate, preventing the pressure signal from reaching the switch.

  5. 05

    Restricted or blocked flue or exhaust vent

    Wiring / connection

    A blockage in the exhaust vent pipe, such as a bird nest, debris, or collapsed duct, can create back pressure that reduces the blower's ability to move air and actuate the switch. Inspect the full vent path for obstructions.

See the test procedure for each cause

Frequently asked questions about 10

What does 10 mean on a A.O. Smith water_heater?
Error code 10 means the air pressure switch contacts did not close within 5 seconds of the combustion fan being energized. This tells the control board that proper airflow has not been confirmed, so the ignition sequence is halted as a safety precaution. The fault is most commonly caused by a clogged combustion air intake screen, a failing blower motor, or a defective pressure switch.
How do I fix error code 10 on my A.O. Smith water heater?
Start by locating and cleaning the screen on the combustion air intake chamber, as debris buildup is the most frequent cause. After cleaning, reset the heater and observe whether the blower starts and the fault clears. If the error returns, inspect the pressure switch tubing for cracks or disconnections, test the pressure switch for proper continuity, and check the blower motor for weak or erratic operation.
Is error code 10 on an A.O. Smith water heater dangerous?
The fault code itself is a protective lockout, meaning the heater has stopped operation to prevent ignition without confirmed airflow. This is the safe state. However, the underlying cause, such as a blocked vent or failed blower, should be diagnosed and corrected promptly, since these conditions could allow combustion gases to accumulate if the heater were somehow operated without proper ventilation.
Will resetting my A.O. Smith water heater clear error code 10?
A manual reset will clear the lockout and allow the heater to attempt another startup cycle, but if the underlying cause is not corrected, the code will return within seconds once the blower energizes again. Cleaning the air intake screen before resetting gives the best chance of a successful restart without the fault recurring.
How much does it cost to repair error code 10 on an A.O. Smith water heater?
If the intake screen cleaning resolves the issue, the repair costs nothing beyond your time. Replacing the air pressure switch typically runs in the range of $20 to $80 for the part, depending on the model. A blower motor replacement is more involved and can range from $100 to $300 or more for parts, with additional labor costs if a technician is needed.

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