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Error code reference

A.O. Smith Water Heater Error Code 6

Air pressure switch contacts remain open longer than 11 seconds after the combustion fan is energized.

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

Air pressure switch contacts remain open longer than 11 seconds after the combustion fan is energized.

First step from the service manual

Clean the screen on the combustion air intake chamber, then turn power OFF for 10-20 seconds then ON to clear error code.

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

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What 6 means on a A.O. Smith water heater

Error code 6 on an A.O. Smith water heater indicates that the air pressure switch contacts have remained open for more than 11 seconds after the combustion fan has been energized. The air pressure switch is a safety component that monitors negative pressure (draft) inside the combustion air chamber. When the fan spins up, it should create enough suction to pull the switch contacts closed within a few seconds, confirming adequate airflow for safe combustion. If the contacts do not close within the 11-second window, the control board locks out and displays this fault to prevent unsafe operation.

The most common trigger for this fault is a blocked or restricted combustion air intake screen. Dust, lint, debris, and spider webs frequently accumulate on this screen and reduce airflow enough that the fan cannot generate sufficient negative pressure to close the switch. This is why the first diagnostic step is to clean the intake screen and then power-cycle the unit for 10 to 20 seconds to reset the fault.

If cleaning the intake screen does not resolve the fault, the issue may be mechanical or electrical in nature. The air pressure switch itself can fail, leaving its contacts permanently open regardless of actual draft conditions. The pressure sensing port or rubber tubing connecting the switch to the combustion chamber may also be blocked or cracked, preventing accurate pressure readings. Finally, the combustion fan motor may be running below its rated speed due to wear or electrical issues, producing insufficient suction even with a clean intake path.

Source: manufacturer service documentation.

Common causes of 6

  1. 01

    Blocked combustion air intake screen

    Wiring / connection

    Lint, dust, debris, or spider webs on the intake screen restrict airflow enough that the fan cannot build sufficient suction. Visually inspect and clean the screen as the first step before any other diagnosis.

  2. 02

    Failed air pressure switch

    Part replacement

    The air pressure switch contacts may be stuck open or the switch diaphragm may have failed, preventing the switch from closing even when draft is adequate. A multimeter can be used to check continuity across the switch contacts while the fan is running.

  3. 03

    Blocked or damaged pressure sensing tube

    Wiring / connection

    The small rubber or plastic tube connecting the air pressure switch to the combustion chamber can become kinked, cracked, or clogged with debris, preventing the switch from sensing actual pressure. Inspect the tubing for physical damage and clear any obstructions.

  4. 04

    Weak or failing combustion fan motor

    Part replacement

    If the fan motor is worn or receiving insufficient voltage, it may spin too slowly to generate the negative pressure required to close the switch contacts within the 11-second limit. Listen for unusual noise from the fan and verify the motor is reaching full speed.

  5. 05

    Blocked flue or venting restriction

    Wiring / connection

    An obstruction in the exhaust venting path can prevent proper draft from developing even if the fan and intake screen are in good condition. Inspect the vent pipe for blockages, bird nests, or collapsed sections.

See the test procedure for each cause

Frequently asked questions about 6

What does 6 mean on a A.O. Smith water_heater?
Error code 6 means the air pressure switch contacts stayed open for more than 11 seconds after the combustion fan was energized. This tells the control board that sufficient airflow or draft was not confirmed within the required time window. The heater shuts down as a safety precaution until the airflow issue is resolved. The most common cause is a clogged combustion air intake screen.
How do I fix error code 6 on my A.O. Smith water heater?
Start by cleaning the screen on the combustion air intake chamber to remove any dust, lint, or debris. After cleaning, turn power off for 10 to 20 seconds and then restore power to clear the fault and allow the heater to attempt a new ignition cycle. If the code returns, inspect the pressure sensing tube, check the air pressure switch for continuity, and examine the venting path for blockages. Persistent faults after these steps typically indicate a failed pressure switch or combustion fan motor that needs replacement.
Will resetting my A.O. Smith water heater clear error code 6?
A power reset by turning the unit off for 10 to 20 seconds and back on will clear the fault code and allow the heater to retry. However, if the underlying cause such as a blocked intake screen or failed pressure switch has not been corrected, the code will return during the next startup attempt. Always address the root cause before relying on a reset.
Is error code 6 on an A.O. Smith water heater a safety concern?
Yes, the air pressure switch is a safety device designed to confirm that combustion air and draft are present before allowing the burner to fire. When the heater displays code 6 and shuts down, it is functioning correctly as a safety measure. Do not attempt to bypass or override the air pressure switch, as doing so could allow combustion without adequate ventilation, creating a carbon monoxide hazard.
How much does it cost to repair error code 6 on an A.O. Smith water heater?
If the fix is simply cleaning the intake screen, there is no parts cost involved. If the air pressure switch needs replacement, parts typically range from $20 to $60 depending on the model, with professional labor adding $75 to $200 or more depending on local rates. A failed combustion fan motor is a more expensive repair, with parts and labor potentially ranging from $150 to $400. Diagnosing the exact cause before ordering parts can help avoid unnecessary expenses.

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