Tankless Water Heater Error Codes: Every Brand Compared

A tankless water heater is a burner, a heat exchanger and a nervous system: flow, temperature, airflow and flame sensors reporting to a control that fires, modulates and shuts down in seconds. Its error codes are that nervous system talking — more numerous, more specific and more honest than a tank heater's, because the machine supervises more of itself. This page explains how tankless reporting differs from tank reporting, translates the faults the big five systems share, and covers the smaller and RV brands whose codes follow the same physics under sparser documentation.

How tankless codes differ from tank codes: a supervised burner

A tank heater's control watches a short chain — ignition, flame, temperature — over minutes. A tankless control runs the same chain in seconds, every time a tap opens, and watches far more links: water flow rate and its changes, inlet temperature, outlet temperature at the heat exchanger and at the outlet, fan speed against commanded speed, airflow proven before ignition and maintained during the burn, exhaust temperature, condensate level on condensing models, and the gas valve's modulation as it tracks demand. Each watched link gets codes, which is why a Navien or Bosch list runs to dozens of entries where a tank control has a handful of flash patterns. The granularity is genuinely useful: a tankless code often separates 'the fan never proved airflow' from 'airflow was proven but ignition failed' from 'flame lit and then died' — three different repairs that a tank control compresses into one ignition pattern.

Tankless reporting also has a category tank reporting barely knows: the derate and the maintenance report. A derate (Bosch's AD is the documented type) is the unit announcing reduced capacity — it still heats, at lower maximum power, because a condition (airflow margin, altitude, vent length) is outside its comfort zone. Maintenance reports announce consumption, not failure: scale building in the heat exchanger (signalled directly by flue-temperature codes on Bosch, by service reminders on others), an inlet filter due for rinsing, a condensate neutralizer due for replacement on brands that monitor it. Reading tankless codes well means sorting every report into fault, derate or maintenance before reacting — the three classes want a service call, a watchful note, and a cleaning respectively, and confusing them wastes both money and alarm.

Flow first: minimum-flow behaviour and the codes around it

Every tankless heater has an activation threshold — a minimum flow, roughly half a gallon per minute on many residential models (Bosch's Therm documentation states 0.5 gpm; the Greentherm 9000 activates at 0.45 gpm) — below which the burner will not fire at all. This produces the most misdiagnosed non-fault in tankless ownership: a tap opened a crack, a low-flow fixture, or cold water mixed in so heavily at a balancing valve that hot flow through the heater drops under the threshold, and the water runs cold with no code, because as far as the heater is concerned there is no demand. The companion behaviour is the cold-water sandwich and the mid-draw dropout, where flow hovering at the threshold cycles the burner. None of these is a code fault — they are hydraulics — and the test is always the same: open one hot tap fully and judge the heater there, not at a trickling fixture.

When flow does generate codes, they split by direction. Too little proven flow during a burn overheats the exchanger and surfaces as temperature codes; flow sensed when there should be none (a crossover, a leaking fixture, a recirculation fault) surfaces as flow-sensor and recirculation reports (Bosch's C3, flow sensor sees no recirculation, is the scoped example); and flow beyond the design maximum gets its own report on units that watch it (Bosch CA, flow above 9.2 gpm). The owner's flow toolkit is short and external: rinse the inlet filter screen — the single most common flow restriction in the machine, named in every brand's maintenance list — confirm isolation valves are fully open, and check that a plumbing crossover (a failed single-lever mixing valve is the classic) is not pushing cold water into the hot line. Everything downstream of those checks is internal and professional.

Smaller and RV brands: the same physics, thinner documentation

Noritz is the largest of the smaller names and the most systematic: its tankless heaters report two-digit display codes in the same spirit as Rinnai's — the firing sequence and the sensor chain, read on the unit or its remote — and Noritz documents them model by model in its manuals rather than as one universal chart. Owners should expect ignition, flame, fan/airflow and temperature families in the same order as the big brands, and should decode strictly from their own model's manual, because Noritz numbers are its own dialect and do not borrow Rinnai's or Rheem's meanings.

Takagi's codes live a double life: Takagi built tankless heaters under its own name with a numeric display-code system (its three-digit family, in which 111 reports ignition failure), and AO Smith's tankless line shares that platform heritage, which is why AO Smith's 111 and Takagi's documentation rhyme. A Takagi owner and an AO Smith tankless owner are often reading dialects of the same system — but 'often' is the operative word: confirm against the manual for the badge on the unit, because generations differ inside both names.

Girard serves the RV market with tankless units fitted to motorhomes and trailers, where the reporting is deliberately simple — a small display and fault indications aimed at owners on the road rather than technicians with laptops. The RV context changes the fault mix more than the code list: low battery voltage, low propane pressure from a nearly empty or cold tank, and low water pressure from park supplies produce ignition and flame behaviour that the same unit, fed properly, would never show. On any RV tankless, check the supplies — propane, 12-volt power, water pressure — before believing the code is about the heater.

Suburban, the other big RV name, builds both tank and tankless RV water heaters, and its reporting follows the RV pattern: fault lights and simple indications on the control and wall switch rather than deep numeric lists, with lockout behaviour after failed ignition attempts that resets by cycling the switch. As with Girard, the diagnosis order is supplies first — propane pressure, battery state, water pressure and flow — because an RV heater lives at the end of three utilities that all sag, and its sparse reporting assumes the owner will check them.

State is AO Smith's sibling badge in the State Industries family, and its tankless offerings follow the same reporting approach as AO Smith's — display codes on tankless models, control-based indications on tanks — documented per model rather than as a single State chart. The working rule is the family rule: decode a State unit from the manual for its platform, and expect the tankless numbers to behave like the AO Smith/Takagi dialect they descend from, verified against the State document itself before acting.

Westinghouse tankless units (sold through big-box channels, built on licensed platforms) report with numeric display codes in the common tankless families — ignition, flame, airflow, over-temperature — but the brand's documentation is thinner and more model-scattered than the big five's, and third-party Westinghouse tables routinely import numbers from the underlying platform's other badges. The honest method is strict: the manual that shipped with the unit, or the maker's support library for the exact model, is the only table to trust.

HTP builds high-efficiency tankless and combi units whose controls report display codes and, on its communicating models, fuller text descriptions of the faulted subsystem. HTP's documentation is model-manual based like Noritz's, and its condensing designs make the condensate and venting families more prominent than on non-condensing units. Decode from the model's own manual; HTP codes are a dialect of their own and are not interchangeable with the brands its units are sometimes compared against.

Jacuzzi's tankless line (a licensed-badge programme sold through home-improvement channels) uses numeric display codes in the standard families, with documentation that follows the underlying platform. Because the badge has covered more than one platform over its life, the model number matters even more than usual: two Jacuzzi-badged heaters of different generations may not share a table. Find the platform manual for the exact model, decode there, and treat any universal 'Jacuzzi code list' with the suspicion this site applies to all unscoped lists.

Navien water heater error codes

Navien's tankless reporting is the most granular of the big systems: E-prefixed three-digit codes (E003 ignition failure, E110 air-pressure/exhaust abnormality) that name subsystems individually — flow, each temperature sensor, fan, condensate, recirculation — and surface identically on the unit display and in its app. For a tankless buyer this precision is the point: a Navien code is often a component accusation, not just a failed step. Scope still applies inside the brand: NPE, NPN and combi-boiler generations overlap heavily without being identical, so the model belongs in every lookup.

Full list: Navien Water Heater Error Codes List — every verified Navien code with causes and first fixes.

Rinnai water heater error codes

Rinnai's two-digit tankless codes put the firing sequence first — 10 air supply or exhaust blockage, 11 no ignition, 12 flame failure — and that ordering is a diagnosis habit worth copying: prove air before blaming ignition, prove ignition before blaming flame sensing. Codes read on the unit or remote, lockout behaviour is documented per code, and Rinnai's tables distinguish warnings the unit can run through from faults that stop it. Rinnai numbers stay inside Rinnai: the overlap with Rheem's 11/12 is convergent, not shared.

Full list: Rinnai Water Heater Error Codes List — every verified Rinnai code with causes and first fixes.

Rheem water heater error codes

Rheem's tankless dialect is two-digit display codes with the combustion teens at its centre (11 ignition failure, 12 flame failure) and airflow, exhaust, flow and temperature families around them, read on the unit or its remote control. Rheem publishes its tankless chart per platform generation, and its condensing and non-condensing lines differ in the condensate and exhaust-temperature codes they can produce — one more case where the model, not the brand, owns the table. Rheem's tank flash-pattern language is a separate system entirely and never mixes with these numbers.

Full list: Rheem Water Heater Error Codes List — every verified Rheem code with causes and first fixes.

AO Smith water heater error codes

AO Smith's tankless heaters use three-digit display codes — 111 ignition failure is the head code — in the platform dialect they share heritage with (Takagi). The system walks the firing sequence and the sensor chain in three-digit steps, shown on the unit's display. AO Smith tankless ownership has one documentation quirk: because the platform is shared, Takagi-flavoured tables circulate for AO Smith units and are often right in outline — but the authority is AO Smith's manual for the model, and any code not in it should be treated as unverified, however confident the borrowed table sounds.

Full list: AO Smith Water Heater Error Codes List — every verified AO Smith code with causes and first fixes.

Bosch water heater error codes

Bosch's Greentherm 9000 is the most sensor-dense reporter in this group: letter families (A sensors, C air/vent/fan/valves, E temperature cut-offs and flame, F control and gas path) with derate codes (AD), calibration codes (BB/BC) and a failure-history menu holding the last 10 codes — a system designed to be read by a technician with the manual open, which is exactly how Bosch's table is written (most solutions are marked licensed-professional only). The older Therm generation shares the alphabet with different meanings at A2/A3/A4/A7/E3/E4, so Bosch tankless codes are always generation-scoped on this site.

Full list: Bosch Water Heater Error Codes List — every verified Bosch code with causes and first fixes.

Bradford White water heater error codes

Bradford White's tankless presence is its Infiniti line, and it breaks the brand's tank habit: where Bradford White tanks speak only in gas-control flash patterns, the Infiniti tankless models use display codes in the modern manner. That makes Bradford White the cleanest illustration of this page's platform rule — the badge does not choose the language, the machine type does. Decode an Infiniti from its own manual's display-code table, and never carry its numbers back to a Bradford White tank, which has no numbers to receive them.

Full list: Bradford White Water Heater Error Codes List — every verified Bradford White code with causes and first fixes.

Same fault, five displays

Descaling and maintenance codes: the reports that are not failures

A tankless heat exchanger is a narrow water path heated hard, and hard water scales it — every brand's documentation says so, and several build the warning into the code system. Bosch is the most explicit: its manuals name specific codes (AA, E3, A5 among them) as scale signals, because scale insulates the water side and the flue side runs hotter to compensate, so a flue-temperature code in a hard-water home is a descaling appointment wearing a fault's clothes. Other brands surface the same physics as service reminders, temperature-instability complaints, rumbling (kettling) noises, or flow codes as the scaled passages narrow. The owner's response is maintenance, not parts: a pump descaling with the manufacturer's procedure (a licensed job on most installations, an owner job only where the brand and the isolation valves allow), plus the inlet filter rinse that should accompany it.

The maintenance family is wider than scale. Condensate neutralizers exhaust their media and are monitored or scheduled on condensing models; fan air filters clog in dusty or pollen-heavy air and present as airflow codes (Bosch sells a pleated filter for exactly this and groups it with its C-code checks); inlet screens catch debris after any plumbing work upstream. The reading skill is temporal: a code that arrives after years of silent service, in a hard-water postcode, with gradually declining flow or rising noise, is a maintenance report until proven otherwise — while the same code on a new installation is an installation fault (venting, gas sizing, condensate routing) until proven otherwise. Age and water hardness are diagnostic facts; give both to any technician you call.

When a tankless code means stop

Most tankless codes are the machine declining one call for hot water. A short list means stop using the heater until a technician has seen it. Gas-path codes — Bosch's FA (gas leakage in the gas path) is the archetype — end all operation: shut the gas cock, no resets. Over-temperature codes that return immediately after cooling and one reset describe a control or flow failure that can scald, and the water should stay off skin. Exhaust-temperature and cabinet-temperature codes that persist after the vent terminations are proven clear describe possible internal flue leakage or a failing heat exchanger — carbon-monoxide territory, professional-only. And any code accompanied by the physical evidence — gas smell, soot, water leaking from the unit's body, a CO alarm — is an emergency first and a code second.

Lockouts deserve a final word, because owners read them as the machine giving up. A lockout is the control's memory made visible: it has watched the same step fail repeatedly and has stopped offering the attempt. Clearing a lockout without changing anything teaches the control nothing and, on gas appliances, repeatedly filling a burner with unburned gas is the specific harm the lockout exists to prevent. The productive response to any lockout is the record — code, model, the stage it failed at, what changed recently (weather, gas work, plumbing work, a long idle period) — handed to a technician, for whom a lockout plus its history is a head start rather than a dead end.

How to diagnose any 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.

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