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Error code reference

A.O. Smith Water Heater Error Code 11

Flammable Vapor Sensor is open.

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

Flammable Vapor Sensor is open.

First step from the service manual

Check all wiring to the flammable vapor sensor.

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

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  • Wiring diagram references and connector pinouts from the OEM service manual
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What 11 means on a A.O. Smith water heater

Error code 11 on an A.O. Smith water heater indicates that the flammable vapor sensor (FVS) circuit is open. The flammable vapor sensor is a critical safety component designed to detect the presence of flammable vapors near the water heater. When the control board reads an open circuit from this sensor, it immediately locks out the burner to prevent a potential ignition hazard. An open circuit means the electrical signal path through the sensor has been broken, which the control interprets as either a failed sensor or a wiring fault.

The flammable vapor sensor operates using a thermistor-based element whose resistance changes in the presence of flammable vapors. Under normal conditions, the sensor maintains a specific resistance within an expected range. When the circuit reads as open, resistance is effectively infinite, which falls outside any acceptable operating parameter. This condition can result from a broken wire, a loose or corroded connector, or a sensor element that has failed internally.

The first step in diagnosing code 11 is a thorough inspection of all wiring connected to the flammable vapor sensor. Check for disconnected harness plugs, broken wire insulation, pinched wires, and corrosion at the connector terminals. If wiring checks out intact and properly seated, the sensor itself should be tested for continuity. A sensor showing no continuity across its terminals has failed and requires replacement to restore normal operation.

Source: manufacturer service documentation.

Common causes of 11

  1. 01

    Loose or disconnected wiring connector

    Wiring / connection

    The harness plug connecting the flammable vapor sensor to the control board may have come loose or fully disconnected, breaking the circuit. Inspect all connectors at both the sensor and control board ends and reseat any that are not fully engaged.

  2. 02

    Failed flammable vapor sensor

    Part replacement

    The sensor element itself can fail internally, resulting in an open circuit even when wiring is intact. Test the sensor for continuity with a multimeter; no continuity across the terminals confirms the sensor has failed and needs replacement.

  3. 03

    Broken or damaged wire in the sensor harness

    Wiring / connection

    Physical damage to the wiring harness, such as a pinched, cut, or broken wire, can open the circuit. Trace the entire wire run from the sensor to the control board looking for visible damage or breaks.

  4. 04

    Corroded connector terminals

    Wiring / connection

    Moisture exposure can cause corrosion at the sensor connector terminals, increasing resistance to the point of an open circuit. Inspect terminals for green or white oxidation and clean or replace the connector as needed.

  5. 05

    Faulty control board

    Part replacement

    If the sensor and all associated wiring test good, a defective control board that cannot properly read the sensor circuit may be responsible. This is the least common cause and should be diagnosed only after all other components are confirmed functional.

See the test procedure for each cause

Frequently asked questions about 11

What does 11 mean on a A.O. Smith water_heater?
Error code 11 indicates that the flammable vapor sensor circuit is open, meaning the control board is not receiving a valid signal from the sensor. The heater locks out the burner when this code appears as a safety precaution. The fault can stem from a wiring problem, a failed sensor, or corroded connector terminals. A technician should inspect the sensor wiring and test the sensor before attempting to restore operation.
Can I fix error code 11 on my A.O. Smith water heater myself?
A knowledgeable DIYer can start by checking all wiring connections to the flammable vapor sensor, reseating any loose connectors, and inspecting for visible wire damage. If the wiring appears intact, testing the sensor with a multimeter for continuity is the next step. Sensor replacement is a manageable repair for someone comfortable working with appliance wiring, but if the issue involves the control board, professional service is recommended.
Is error code 11 on an A.O. Smith water heater a safety concern?
Yes, the flammable vapor sensor is a safety device that protects against ignition of flammable vapors near the heater. The unit locks out burner operation when code 11 appears precisely because this safety circuit cannot be confirmed as functional. Do not attempt to bypass the sensor or override the lockout. Address the wiring or sensor fault before returning the heater to service.
Will resetting the water heater clear error code 11?
A reset may temporarily clear the fault display, but code 11 will return if the underlying open circuit condition is not corrected. The control board continuously monitors the flammable vapor sensor, so any persistent wiring fault or failed sensor will trigger the code again after reset. The root cause must be repaired for the heater to operate normally.
How much does it cost to repair error code 11 on an A.O. Smith water heater?
If the fix is simply reseating a loose connector, there may be no parts cost, though a service call fee typically ranges from $75 to $150. Replacing the flammable vapor sensor itself generally costs between $50 and $150 for the part, plus labor if a technician performs the work. If the control board is at fault, parts and labor combined can range from $200 to $400 or more depending on the model and local labor rates.

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