mypros+
Error code reference

A.O. Smith Water Heater Error Code 721

False flame detection

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

False flame detection — flame is detected when there should be none.

First step from the service manual

Shut off all gas and power to the unit immediately; contact Technical Service Department.

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 721.

  • 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 721 means on a A.O. Smith water heater

Error code 721 on an A.O. Smith water heater indicates false flame detection, meaning the flame sensor or control system is registering the presence of a burner flame when no ignition sequence has been initiated and no flame should exist. This is a critical safety fault. The control board continuously monitors the flame sensor circuit to confirm that combustion is occurring only when commanded. When the board detects a flame signal outside of an active ignition or heating cycle, it flags this condition as a dangerous anomaly and locks out the system.

The flame sensor operates by detecting the small electrical current conducted through an active gas flame between the sensor rod and the burner ground. A healthy flame sensor produces a microamp-level signal that the control board reads to confirm combustion. A false flame signal can be caused by a sensor rod that has developed a short to ground, a contaminated or cracked sensor probe, wiring faults in the sensor circuit, or a control board that is misreading the signal due to internal component failure.

Because this fault indicates the possibility of uncontrolled combustion or a serious electrical fault within the burner assembly, the first response is to immediately shut off all gas supply and disconnect power to the unit. This code should not be cleared with a simple reset without identifying and correcting the underlying root cause. A.O. Smith directs technicians and homeowners to contact their Technical Service Department before attempting any further diagnosis or repair on a unit displaying this code.

Source: manufacturer service documentation.

Common causes of 721

  1. 01

    Shorted flame sensor rod

    Part replacement

    The flame sensor probe can develop a short to ground due to carbon buildup, physical damage, or a cracked ceramic insulator, causing it to send a continuous false signal to the control board even when no flame is present.

  2. 02

    Faulty or damaged flame sensor wiring

    Wiring / connection

    Wiring between the flame sensor and the control board can become pinched, melted, or grounded against the burner assembly, producing a false flame signal. Inspect the wire harness for visible damage or contact with hot surfaces.

  3. 03

    Failed gas valve not fully closing

    Part replacement

    If the gas valve fails to fully close after a heating cycle, residual gas can continue to burn at the burner, creating a real but unintended flame that the sensor correctly detects. This is a gas safety hazard requiring immediate attention.

  4. 04

    Control board failure

    Part replacement

    An internal failure in the control board can cause it to misinterpret the sensor input circuit and register a false flame signal. If all external components and wiring check out, the control board itself may need replacement.

  5. 05

    Contaminated or cracked sensor probe

    Part replacement

    Exposure to silicone vapors, excessive moisture, or physical stress can contaminate or crack the flame sensor probe, altering its electrical characteristics and causing erroneous readings.

See the test procedure for each cause

Frequently asked questions about 721

What does 721 mean on a A.O. Smith water_heater?
Error code 721 indicates false flame detection, meaning the water heater's control system is sensing a flame when the burner should be off. This is a serious safety fault that requires immediate action. You should shut off all gas and power to the unit and contact A.O. Smith's Technical Service Department before attempting any repair.
Is it safe to keep using my water heater when error code 721 is displayed?
No, it is not safe to continue operating the unit while code 721 is active. This error can indicate an uncontrolled flame, a stuck-open gas valve, or a critical electrical fault. Shut off the gas supply and disconnect power to the water heater immediately and do not use it until the cause has been diagnosed and corrected by a qualified technician.
Will resetting my A.O. Smith water heater clear error code 721?
A simple power reset is unlikely to permanently clear code 721 because the underlying fault, whether a shorted sensor, faulty wiring, or control board issue, will still be present when power is restored. Even if the unit temporarily restarts, the code will return. The root cause must be identified and repaired before the system will operate safely and reliably.
Can I fix error code 721 myself?
A.O. Smith specifically directs users to contact their Technical Service Department as the first step for this code, which reflects the seriousness of a false flame detection fault. While a knowledgeable technician can inspect the flame sensor, wiring harness, and gas valve, this is not a repair recommended for inexperienced DIYers due to the gas and electrical safety risks involved.
How much does it cost to repair error code 721 on an A.O. Smith water heater?
Repair costs vary depending on the failed component. Flame sensor replacement is typically the least expensive fix, often in the range of $100 to $250 including parts and labor. A faulty gas valve or control board replacement will cost more, generally ranging from $250 to $600 or higher depending on the model and local labor rates. A service technician will need to diagnose the specific cause before an accurate estimate can be provided.

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