AO Smith Water Heater Fault Codes
Every documented AO Smith tankless and tank water heater error code, what it means, and when it needs a professional.
AO Smith splits its fault-reporting into two different systems depending on the product line. Gas storage tank models with electronic ignition (ProLine, Signature, and similar residential FVIR tank units) report problems through a small red status light that blinks a set number of times, pauses, and repeats — no digits, just a count. Tankless and condensing models (mid- and high-efficiency indoor and outdoor units) instead show a numeric code, either on the unit's built-in display or on a wired remote temperature controller. Both systems point to the same underlying idea: the control board detected something specific and is telling you what, if you know how to read it.
How to read the codes
On a tank model, count the blinks between pauses — two flashes, a pause, two flashes again, and so on — and match that count to the table below; there's no way to mistake a two-flash code for a four-flash one if you count through a full cycle rather than catching it mid-blink. On a tankless unit, the code is just printed as digits (like 111 or 991) on the display or remote, sometimes alongside a smaller sub-code that narrows down the specific sensor or component involved. If you're not sure which product family you own, check the rating plate on the tank or unit cabinet, or the model number in your owner's manual.
All AO Smith codes
- 6 Flashes — Flammable vapor sensor lockout
- 4 Flashes — High-temperature limit (ECO) trip
- 2 Flashes — Low thermopile voltage
- 7 Flashes — Gas control valve failure
- 111 — Ignition failure
- 121 — Loss of flame
- 991 — Imperfect combustion
- 611 — Fan motor fault
- 701 — Control board fault
- 741 — Remote controller miscommunication
No code showing — just a symptom?
If the status light isn't blinking at all, or you haven't found it yet, start from what's actually happening instead: ignition failure, pilot won't light or won't stay lit, no hot water from a tank unit, or no hot water from a tankless unit.