A.O. Smith Water Heater Error Code 121

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.

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.

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.

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.

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.