Error code reference

A.O. Smith Water Heater Error Code 121

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

SourceA.O. Smith 710/710 ASME - Service Manual: Troubleshooting & Maintenancep. 14

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

Diagnostic procedure

The full 121 procedure

Step 1 is visible so you can confirm you are on the right page. The remaining steps, the ones that tell you whether to actually replace a part, unlock with any pass.

MyPros+ · DiagnosticsStep 1 preview

First diagnostic step

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

SourceA.O. Smith 710/710 ASME - Service Manual: Troubleshooting & Maintenancep. 14

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More steps, wiring diagram references, and verified part numbers

Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.

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Where this code is documented

1 manual in the MyPros+ library documents this code for A.O. Smith water heater.

Models covered by the manuals that document this code

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The full 121 procedure

Where this code is documented

Models covered by the manuals that document this code

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