AO Smith Tankless Error Codes by System (101, 111, 121, 711, 991, E02, E03, E21)

AO Smith's gas tankless units — the ATI/ATO line built on Takagi's platform — speak in three-digit codes, shown in full on the unit display, shortened by one digit on the remote controller, and repeated as red-LED blinks on the computer board. This system article gives the combustion codes 101, 111, 121, 711 and 991 the full treatment, then covers the three E-codes the tankless family also produces — E02, E03 and E21, which belong to AO Smith's electric tankless models and are scoped as such — with the diagnosis order a technician actually follows at the end.

One platform, three ways of saying the same code

Before the codes, learn the platform's accent, because AO Smith tankless units report every fault three ways and owners routinely fail to recognise their own code in a chart. The built-in display and the computer board show the full three-digit number: 111, 121, 991. The remote controller drops the final digit, so the same faults read 11, 12 and 99 on the remote — AO Smith's own service guide is written in the remote's two-digit dialect, which is why its sections are headed 11, 12, 31, 32, 39 and 99. The third channel is the red LED on the computer board, which repeats the fault as a blink count grouped by family: once for computer and gas-valve-drive faults, twice for sensor and rod faults, three times for ignition and flame faults, four times for fan and valve faults, five times for combustion faults, six times for gas solenoid and false-flame faults. A complete report of any event on this platform is therefore three numbers that must agree — display 111, remote 11, three blinks. The platform itself is Takagi's: AO Smith's tankless line is built by its own tankless company, and the code list, part numbers and diagnostic flowcharts are shared between the two brands' manuals. What follows uses AO Smith's wording, cross-checked against Takagi's documents for the same hardware. Every combustion code here shares one boundary: the owner checks supplies and vents from outside the cabinet, and everything inside the burner and gas train belongs to a licensed technician.

Code 101: the warning shot before 991

Code 101 is documented in AO Smith's guide as the warning for the 991 error code — the unit has detected combustion drifting out of specification and is telling you before the drift becomes a shutdown fault. On the remote it shows as 10, and the board's red LED blinks five times, placing it in the combustion family. (The older Takagi chart words 101 as insufficient gas supply; read the two wordings together, because inadequate gas delivery is one of the main ways combustion goes abnormal on this platform.) AO Smith's diagnosis list for the warning is the full combustion audit, in a sensible order. Check the gas type first: a unit plumbed to the wrong fuel, or converted incorrectly, cannot burn cleanly at any setting. Inspect the intake air and exhaust for blockage — the guide names damper sticking, vent flaps and snow buildup, and the classic installation fault of a unit closet without adequate ventilation. On direct-vent systems, confirm there is enough separation between the intake and exhaust terminals that the unit is not re-breathing its own exhaust. Verify the vent run itself: total length and elbow count within the manual's limits. Check the altitude setting, because a sea-level dipswitch setting at elevation starves the mixture exactly as a blockage does. Only then does the list go internal: grease and dirt in the burner and fan motor, dust and lint in the heat exchanger, and manifold gas pressure measured against the manual. The owner's portion is the external half — terminations clear, closet ventilated, no stored clutter choking the intake. The internal half is a combustion service visit. A 101 that is reset and ignored reliably graduates to 991; treat the warning as the cheap version of the fault.

Code 111: ignition failure

Code 111 means the burner failed to ignite — the unit ran its ignition sequence and never proved flame, so it shut down rather than keep feeding gas to an unlit burner. Remote dialect: 11. Board dialect: three red blinks. AO Smith's service guide works it as an eight-point check, and the order teaches the logic. Begin with supply: is gas actually reaching the unit, at the right inlet pressure, with the supply valve fully open and the line sized for a tankless load — a unit that fires other appliances' leftovers will spark and starve. Check the hi-limit switch, the overheat cutoff on the heat exchanger; an open hi-limit vetoes ignition before it starts, and a tripped one is itself a report worth explaining. Move to the flame circuit: wiring connections and breakages across the igniter, gas valve and rod harnesses, burn marks on the computer board, and soot on the flame rod — a rod coated in soot cannot prove a flame that is actually there, which is why cleaning it is a genuine repair and not a ritual. Then listen, because this platform talks during ignition: a healthy sequence has a buzzing spark from the igniter during combustion preparation and a double clunk as the gas valves open. Silence where the spark should be points one way (igniter and its voltages), silence where the clunks should be points another (valve drive and its voltages), and the guide's appendices give the meter readings for each branch. The owner's checks end at the supply valve and one power reset. Everything after — rod cleaning inside the burner, gas pressure measurement, board voltages — is licensed gas work, and a 111 that repeats after a successful day of operation is usually the flame rod or the gas supply announcing a trend, not bad luck.

Code 121: loss of flame

Code 121 is 111's mid-operation twin: the burner lit and proved flame, then lost it while a call for hot water was still running. Remote 12, three board blinks, and a diagnostic list that deliberately overlaps 111's — AO Smith's guide works both codes through gas supply, hi-limit, wiring, board condition and flame-rod condition, because a flame that drops out is usually one of the same systems failing intermittently instead of completely. The differences are in emphasis. A 121 unit has demonstrated it can light, so pure ignition hardware drops down the list and stability rises: inlet gas pressure that sags when the unit modulates up or when other appliances fire, a flame rod whose signal is marginal and drops as it heat-soaks, a heat exchanger leak dripping onto the burner, or dust and lint in the manifold nozzles making individual burner ports unstable. The guide adds two 121-specific checks accordingly: inspect for heat exchanger leakage, and measure the flame rod's current against its specification — a rod passing at light-off and fading at temperature is the classic 121 story. Behaviour around the fault is diagnostic gold, so notice it before you call: does 121 arrive at high demand (a long shower, two fixtures) or at low demand, after minutes or within seconds, in wind or in calm? Flame loss tied to high fire points at supply and venting; flame loss at random points at the rod and its wiring. One reset to confirm the pattern is reasonable. A unit reset daily for 121 is a unit burning unreliably on a safety circuit's sufferance — the hi-limit and flame-proving systems are doing exactly their job, and the correct response is a service visit, not a faster reset routine.

Code 711: the gas valve drive circuit

Code 711 reports a failure in the gas solenoid valve drive circuit — the electronic path by which the computer board commands the gas valves open. Its blink signature is a single red flash, grouping it with the computer-board family rather than the ignition family, and AO Smith's Owner's Guide handles it with a cross-reference that is easy to miss and important to understand: for 711, refer to the 111 and 121 error codes. That pointer is the manufacturer saying the drive-circuit fault presents through the ignition codes — a valve that is never driven cannot light (111) or cannot stay lit (121) — and that the working diagnosis runs through the same checks: gas supply proven, flame circuit intact, board examined. Field experience on this platform adds the suspect the flowchart starts with: inlet gas pressure and line sizing. A drive circuit blamed for starvation symptoms is a misdiagnosis that replaces boards while the real fault — an undersized gas line shared with a furnace — keeps producing the code; technicians who know this platform prove the gas supply under full fire before they condemn electronics. For the owner, 711 has exactly one move: note whether 111 or 121 accompanied it, confirm the gas supply valve is open, power-cycle once, and stop. Solenoid drive circuits live on the gas train's electrical boundary; testing them means meter work on valve voltages with the unit attempting ignition, which is licensed work by any definition. Quote all three code forms when you call — display, remote and blink count — because on this fault the combination routes the technician's first hour.

Code 991: imperfect combustion

Code 991 is the platform's most serious combustion verdict: the flame is burning, but it is burning wrong — imperfect combustion, with the carbon-monoxide and sooting consequences that phrase implies. Remote 99, five red blinks, and it is the fault 101 exists to warn about; a unit that has shown 101 and now shows 991 has completed a documented deterioration, which is why the service guide simply refers 99 back to the 10 diagnosis and expects the whole list worked, not sampled. Run it in full. Gas type: the wrong fuel or a botched conversion heads the list again because no adjustment fixes it. Air path: blockages in intake or exhaust, sticking dampers, snow, a starved closet, and on direct-vent units the terminal separation that prevents exhaust recirculation. Vent geometry: length and elbows beyond the manual's limits throttle the fan's ability to move combustion air. Altitude: the dipswitch high-altitude function unset at elevation is a standing 991 generator, and a unit that has shown 991 since a move or a board replacement should have its switches read before its burner is condemned. Internal condition: grease and dirt in the burner and fan, dust and lint fouling the heat exchanger — the guide calls out contaminated environments specifically, because a tankless in a laundry or workshop breathes lint for a living. Finally, manifold pressure measured and adjusted to the manual. None of the internal list is owner work, and the safety framing is blunt: imperfect combustion is the failure mode that produces carbon monoxide in occupied space. A 991 unit stays off until a technician has proven combustion with instruments, and a working CO alarm in the mechanical space is not optional equipment.

Codes E02 and E03: the electric tankless dialect

The tankless family has an electric branch, and it speaks a different code set that this guide must scope honestly: E02 and E03 here are from AO Smith's Electric Tankless Use & Care Guide — point-of-use and whole-house electric tankless units with heating chambers, not the gas platform above. E02 means the hardware protection has determined that the heated water exceeded the temperature limit. Note the engineering in that wording: electric tankless units carry two over-temperature judgements, a hardware one (E02) and a software one (E06), and E02 is the hardware verdict — the manual's action is correspondingly blunt: turn off power to the water heater and contact Technical Support. There is no owner reset procedure printed for E02, and inventing one by cycling breakers until it behaves is exactly wrong for an over-temperature event on a unit that heats water in seconds at full power. E03, by contrast, has a genuine owner remedy because its cause is mundane: low water level in the heater — the chambers are not proven full, almost always air trapped after installation, a plumbing repair, or a supply interruption. The manual's procedure is a purge: turn off power, open a nearby hot water faucet and flush air from the line, wait until water runs uninterrupted from the fixture, then restore power. The sequence matters — power stays off while air is in the chambers, because an electric element energised in air is a dry-fire, the fastest way to kill it. If E03 returns after a clean purge with solid water flow, the level detection itself needs service. Both codes share the electric platform's standing warning: these units are fed by multiple heavy branch circuits, all of which must be off before any cover comes off.

Code E21: a shorted thermistor on the electric tankless

E21 completes the electric tankless trio in this guide, and it is a sensor code with a model-dependent twist that the manual states precisely. E21 means a thermistor is shorted. Which thermistor depends on the chassis: on two- and four-chamber models, E21 names TH-1, the first intermediate chamber sensor; on single-chamber models, the same code names T-Out, the outlet thermistor. Its mirror family is the open-sensor set, E10 through E14, where the same sensors report as open circuits — E20 through E24 are the shorted counterparts, and E21 is simply the first of them. This pairing teaches how the platform thinks: a temperature sensor can fail in two electrical directions, open or shorted, and the code's first digit tells you the direction while the second tells you the sensor. Diagnosis at owner level is intentionally thin. A shorted thermistor means the control is being fed a fixed, false temperature for a chamber it is actively firing; the unit cannot regulate that chamber safely, and no reset changes a short circuit. The manual's route is replacement of the named thermistor through its thermistor replacement procedure — power off at every branch circuit (these units may have two or more), cover off, sensor swapped at its chamber. That is qualified-person work for most owners, and the manual frames its whole troubleshooting section with an electric-shock warning for a reason: multi-circuit 240-volt equipment stays lethal on the circuits you forgot. When you call, the one fact that halves the diagnosis is your chamber count from the rating plate — E21 plus 'four-chamber' names the exact sensor before the technician leaves the shop.

Diagnosis order, and when the tankless goes to a pro

Work any gas tankless fault in the platform's own order and you will rarely replace a wrong part. First, write down all three code forms — display, remote, blinks — plus when the fault appears: at light-off, mid-draw, or at high demand. Second, the supplies, which are the owner's whole job: gas valve open and other gas appliances healthy, water inlet filter screen cleaned (the manual's routine maintenance step, and a genuine cause of flow and flame faults), vent terminations outside visibly clear, intake not buried in stored clutter. Third, one power reset, observed: a code that clears and stays gone for days of normal use was an event; a code that returns within the same draw is a condition. Fourth, pattern-match by family: ignition and flame codes (111, 121, 711) go to gas supply, flame rod and board, in that order of suspicion; combustion codes (101, 991) go to air path, vent geometry, altitude and burner condition; sensor and valve codes go straight to parts testing. The pro threshold on this platform is bright: anything requiring the front cover off around the burner, any gas pressure measurement, any board voltage check, and any code in the combustion family that survives clear vents. Add the absolute stops — smell of gas, a false-flame 721, a hi-limit 141 that repeats — and the rule set is complete. These units modulate a large gas burner in a small box; their codes are written by engineers who assumed a technician would read them, and the safest owner skill is translating the code accurately and knowing where your section of the work ends.

Key takeaways

• Translate before you search: remote 11 is display 111, and the board's three red blinks confirm it — quote all three forms when you call for service.
• 101 is 991's warning, not a separate minor fault: work the air path and venting when 101 appears and you may never meet 991.
• Clean the cold-water inlet filter screen before blaming electronics for any ignition or flow fault — it is the manual's own first maintenance step.
• On the electric tankless, E03's fix is a purge with the power OFF — never energise chambers that may hold air.
• One reset after fixing an external cause is diagnosis; daily resets of 111, 121 or 991 are a burner asking for a technician.
• Smell gas, or see 721 (false flame) or a repeating 141 (hi-limit): stop, leave the unit off, and call a licensed technician.

Resetting the AO Smith water heater

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.

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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