mypros+
Error code reference

A.O. Smith Water Heater Error Code 121

Flame loss

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

Flame loss — flames disappear while the unit is still supposed to be in operation.

First step from the service manual

Verify that the hi-limit switch is not tripped; turn power off, push the white button in to reset, then test the heater.

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 121.

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

Error code 121 on an A.O. Smith water heater indicates a flame loss condition: the control board has detected that the burner flame has extinguished while the unit was still in an active heating cycle. The gas valve and igniter are commanded to be running, but the flame sensor (also called a flame rod or flame rectification sensor) is no longer detecting a valid flame signal. This is an operational fault rather than an ignition failure, meaning the burner lit at some point during the cycle but then dropped out before the heating demand was satisfied.

The hi-limit switch is the first component to inspect when this code appears. This safety device is designed to trip when water or component temperatures exceed safe thresholds, and a tripped hi-limit will interrupt burner operation mid-cycle, producing exactly the flame loss symptom that triggers code 121. Resetting the hi-limit involves turning off power to the unit, pressing the white reset button firmly until a click is felt, then restoring power and attempting to run the heater.

If resetting the hi-limit does not resolve the issue, the fault likely originates in the gas supply system, the flame sensing circuit, or the venting arrangement. Inadequate gas pressure, a contaminated or cracked flame rod, combustion air problems caused by blocked venting, or a failing gas valve can all cause mid-cycle flame dropout. A failing control board that misreads the flame signal is also possible but less common. Each of these requires systematic testing to isolate the root cause.

Source: manufacturer service documentation.

Common causes of 121

  1. 01

    Tripped Hi-Limit Switch

    Wiring / connection

    The hi-limit switch cuts power to the burner circuit when temperatures rise too high. A tripped switch will cause the flame to drop out mid-cycle and is the first thing to check. Turn off power, press the white reset button until it clicks, then restore power and test.

  2. 02

    Inadequate Gas Pressure or Supply

    Wiring / connection

    Low gas pressure or a partially closed supply valve can sustain initial ignition but cause the flame to collapse during steady-state operation. Check that the manual gas shutoff valve is fully open and have gas supply pressure tested with a manometer if the valve is open.

  3. 03

    Dirty or Faulty Flame Sensor

    Part replacement

    The flame rod can accumulate oxidation or carbon deposits over time, weakening the rectified current signal it sends to the control board. If the board does not detect a strong enough signal, it interprets this as a flame loss even when the burner is still lit. Cleaning or replacing the flame rod typically resolves this.

  4. 04

    Blocked or Restricted Venting

    Wiring / connection

    Obstructed flue pipes or air intake vents reduce combustion air supply, which can starve the burner and extinguish the flame mid-cycle. Inspect the full vent path for blockages such as bird nests, debris, or collapsed flex duct.

  5. 05

    Failing Gas Valve

    Part replacement

    A gas valve that is beginning to fail may not hold consistent gas flow through a full heating cycle, causing intermittent flame dropout. This is typically diagnosed after ruling out supply pressure, sensing, and venting issues, and the valve will need replacement if confirmed faulty.

See the test procedure for each cause

Frequently asked questions about 121

What does 121 mean on a A.O. Smith water_heater?
Error code 121 means the heater experienced a flame loss condition: the burner flame went out while the unit was still in an active heating cycle. The control board detected that the flame signal dropped to zero before the heating demand was met. This is a safety-triggered shutdown that prevents the unit from continuing to operate without a confirmed flame present.
Can I fix error code 121 myself?
Some causes of code 121 are DIY-friendly. Resetting the hi-limit switch requires only turning off power, pressing the white reset button, and restoring power. Checking that the gas supply valve is fully open and inspecting vents for blockages are also tasks most homeowners can handle safely. However, testing gas pressure, replacing the flame sensor, or diagnosing a gas valve issue typically requires a qualified technician with the proper tools.
Is error code 121 on my A.O. Smith water heater a safety hazard?
The code itself represents the heater doing its job correctly by shutting down when the flame was lost. The unit will not continue running without a verified flame, so there is no immediate risk of unburned gas accumulating from a runaway condition. That said, a repeatedly tripping hi-limit switch can indicate an overheating problem that should be diagnosed promptly to prevent long-term damage or a more serious fault.
Will resetting the heater clear error code 121?
Resetting the hi-limit switch may clear the code and restore operation if the switch was the root cause. However, if the underlying problem persists, such as low gas pressure, a dirty flame sensor, or venting restriction, the code will return. A reset that holds for one or two cycles but keeps coming back indicates the root cause has not been addressed and further diagnosis is needed.
How much does it cost to repair error code 121 on an A.O. Smith water heater?
Repair costs vary widely depending on the cause. If only the hi-limit switch needs resetting, there is no parts cost and only a service call fee if a technician is involved, typically in the range of $75 to $150. Replacing a flame sensor is generally a lower-cost repair, often $100 to $250 in parts and labor combined. A gas valve replacement is more involved and can range from $250 to $600 or more depending on the model and local labor rates.

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