AO Smith Water Heater Error Codes List: Every Code Explained

Every AO Smith water heater error code we could verify against AO Smith's own documentation — what it means, what usually causes it, and the first fix to try — in one table. Codes differ between model series, so treat your model's tech sheet as the final word.

How to read and retrieve error codes on AO Smith water heaters

Start by identifying which reporter your heater owns, because AO Smith uses four unrelated languages. A gas tankless unit (ATI/ATO) shows a three-digit code on its display or remote — the remote drops the last digit, so remote 11 is LED 111 — and repeats the fault as red-LED blinks on the computer board. A smart tank electric or Voltex MAX hybrid shows a three-digit code on its Control Assembly, with a flashing alert indicator and, for serious faults, a beeper; its chart runs from dry-fire 001 through element codes 021/022 to the heat-pump family 080-086. An older Voltex hybrid speaks in words and letter codes on its display — HEAT PUMP CONNECT FAULT, ECC, E10 — a different vocabulary for some of the same hardware. A standard electric with the Electronic Thermostat shows E01-E05 as counted red flashes beside the code, and a gas tank with the knob-type electronic control has no display at all: its status light flashes in counted groups, one flash every three seconds meaning normal. Quote the code with its platform and model — '111' alone is ignition failure only on the tankless chart.

The complete AO Smith water heater error code list

How to reset / clear error codes on AO Smith water heaters

AO Smith documents a different reset for each platform, and none of them is 'keep cycling it until it behaves'. Tankless units reset by power: turn the unit off, restore power, and let it retry — the fault returns at once if its cause (gas pressure, venting, a soot-covered rod) is still present, which is the diagnosis, not a second fault. Smart Control Assembly units power-cycle at the breaker per the chart's 'Power Cycling the Unit' procedure; element and sensor codes persist until the part is fixed, while the ECO behind an over-temperature event is reset at the upper access panel with power off. Tank electric models with conventional or ET thermostats use the red high-limit reset button behind the upper access panel: power off at the breaker, press until it clicks, restore power — a button that trips again has a cause, usually a failed thermostat or grounded element. The knob-type gas control resets by relighting the pilot per the label, with its built-in ten-minute wait between attempts; the powered Intelli-Vent control clears most codes by turning power off for 10-20 seconds, and its flammable-vapor lockout needs the documented two-button Vacation-LED sequence. One reset after a fixed cause is service; repeated resets are how protected heaters get damaged.

How to diagnose any AO Smith water heater code

Work the problem in this order — and stop wherever gas piping, the burner compartment, venting or the temperature-and-pressure relief valve begins, because those belong to a licensed technician: (1) record the report exactly — a counted LED flash pattern, a numeric display code, or a smart-control alarm are three different languages, and a photograph or video beats a memory of any of them; (2) establish the water story: is there no hot water at all, water that runs cold mid-draw, or water but with a code — tankless codes split into ignition, flame, airflow/venting, flow and temperature families, and tank codes into ignition, sensor and control families, and the family picks the checks; (3) run the owner-level external checks for that family first: power and the unit's switch, the gas cock fully open, intake and exhaust terminations clear of snow, nests and debris, the inlet water filter or screen rinsed (tankless), the condensate trap and drain not blocked or frozen (condensing models), and on tank models the status light read before anything is reset; (4) reset once, the way the brand documents, and run hot water at one tap while you watch — a code that returns at the same stage (fan start, ignition click, flame light, sustained burn) is an active fault naming its stage, not a glitch; (5) everything past that line — flame sensors and igniters inside the burner, gas valves, heat exchangers, control boards — is professional work with the gas supply and a meter involved. Two standing cautions: never plug or cap a dripping T&P relief valve (it is reporting pressure or temperature, and it is the tank's last safety device), and treat any gas smell as an emergency — leave and call the gas utility. Scalding is the quiet hazard in every reset: a control fault that lets outlet temperature drift high is a reason to stop using the water on skin until the code is resolved.

The Takagi connection: why AO Smith tankless codes look Japanese-engineered

AO Smith's residential gas tankless line — the ATI indoor and ATO outdoor models, and the Signature and ProLine XE tankless units sold through retail — is built on Takagi's platform. Takagi is AO Smith's own tankless company, and the kinship is visible in every detail of the diagnostics: the three-digit code list (111 ignition failure, 121 flame loss, 991 imperfect combustion), the remote controller that drops the last digit and shows two digits instead, the red LED on the computer board that repeats the code as a blink count, and even the part numbers in the service guide, where the computer board is Part #701 and the gas valve assembly is Part #102 in both brands' documents. This is why Takagi manuals legitimately verify AO Smith tankless codes: they are the manufacturer's own documents for the same hardware. The boundary matters just as much. Takagi documentation says nothing about AO Smith tanks, hybrids or gas controls, and using a Takagi chart on a tankless unit from a different maker — or an AO Smith tank chart on a Takagi-built unit's faults — produces confident, wrong answers. Match the document to the platform, not to the shape of the code.

One brand, four display languages — and the collisions are real

The single most useful fact about AO Smith diagnostics is that the company never standardised its fault languages, so the same string changes meaning as you cross platforms. Code 031 on a tankless unit is a dipswitch setting fault; 031 on a smart Control Assembly is a water leak. E02 on an electric tankless is a hardware over-temperature shutdown; E02 beside the Electronic Thermostat's LED is the tank high-limit event; and E02's cousin on the older Voltex table does not exist at all, because that platform reports its compressor as ECC and its coil sensor as E10 — while E10 on the electric tankless means the inlet sensor is open. Even generations collide: the older Voltex speaks words (HEAT PUMP CONNECT FAULT) where the Voltex MAX speaks numbers (084) for the identical compressor failure, and the gas tank in the corner reports everything as counted light flashes because it has no display. The working rule is absolute: never look up an AO Smith code until you have named the platform that produced it. A code without a platform is not a diagnosis; it is a guess wearing a number.

Limp Mode: the smart platform's quiet second language

AO Smith's smart Control Assembly adds something the older platforms never had: a formal degraded mode. When the control decides a component has failed, it does not simply stop — it reroutes the work. Limp Mode 1 covers an upper element or upper thermistor failure by letting the lower element satisfy the call for heat. Limp Mode 2 mirrors it for the lower side. Limp Mode 3 is the hybrid's version: the heat pump is judged unavailable — a sensor in the 025-028 family, low voltage on 009, condensate on 081, a compressor fault on 083-086 — and both resistance elements take over heating entirely. This is why a Voltex MAX can show a serious code and still deliver hot water, and why owners sometimes ignore 084 for months: the shower stays hot while the electric bill quietly doubles, because resistance heat costs several times what the heat pump costs to run. A limp-mode code is therefore an efficiency emergency rather than a comfort emergency — worth fixing promptly, on a planned visit, with the code number quoted exactly as displayed.

Less common and model-specific codes

Search data shows owners also meet these codes on AO Smith water heaters: Heater. They are not part of the core code list AO Smith publishes for most models — several are series-specific, abbreviated display readings, or codes carried over from a sister platform — so we do not publish meanings for them rather than guess. Treat them by symptom: note what the machine was doing when the code appeared (filling, draining, heating, spinning), run one full power cycle (5 minutes off), and if the code returns, match the symptom to the closest documented family in the table above and check the tech sheet stored inside your machine for the model-specific definition.

Related guides

Facts on this page were checked against manufacturer documentation: www.manualshelf.com · manuals.plus · www.takagi.com · www.takagi.com · www.Takagi.com · manuals.plus · www.hotwater.com · www.manualshelf.com · www.manualslib.com · www.manualshelf.com · www.manualslib.com · www.manualslib.com · manuals.plus · www.hotwater.com