AO Smith 121: Loss of Flame

Hot water was flowing fine, then it wasn't — the burner lit, ran for a stretch, and the flame dropped out mid-cycle, leaving the display showing 121. That's a different problem than an ignition failure, and it points to something disrupting an already-running flame rather than preventing one from starting.

Applies to: AO Smith tankless/condensing water heaters

What it means

Loss of Flame — the burner ignited successfully but the flame was lost during operation. Because the flame was there and then wasn't, the causes tend to involve something interrupting combustion mid-run — a pressure dip, a dirty flame rod losing its signal, or even a developing heat exchanger leak — rather than the startup issues that usually sit behind an ignition-failure code.

Most common causes

• Gas supply pressure issue
• Hi-limit switch fault
• Sooted or damaged flame rod/wiring
• Heat exchanger leak
• Dirty manifold nozzles

What you can safely check yourself

1. Check gas supply is steady
2. Note whether you see any signs of leaking near the unit (moisture, mineral staining)

What the fix costs

A repeat flame-loss diagnosis generally falls in the same $150–$600 range Angi and Fixr quote for tankless water heater service calls, with the actual number depending on whether it's a quick flame-rod cleaning (closer to $75–$250) or a heat exchanger leak, which is a considerably larger repair sometimes closer in cost to replacing the unit outright. Ask the technician directly whether a leaking heat exchanger is suspected before authorizing a repair, since that changes the cost picture significantly.

Related codes

• AO Smith 111 — Ignition Failure
• AO Smith 991 — Imperfect Combustion
• AO Smith 701 — Control Board Fault

All AO Smith codes

Every AO Smith water heater fault code — the full list on one page. See no hot water from a tankless water heater for other reasons hot water can drop out mid-use.

Sources

• AO Smith tankless water heater error code guide (manufacturer documentation)