A.O. Smith Tankless Water Heater Error Codes: The Complete Guide

By the Certified Technicians at THE Water Heater Company
Licensed Water Heater Specialists — Every Technician A.O. Smith Certified Through A.O. Smith University | Three Locations Serving Ventura County, Los Angeles County, Orange County & Santa Barbara Since 2018
Last Updated: July 2026

Your A.O. Smith Is Talking to You. Here's How to Understand What It's Saying.

You checked your A.O. Smith tankless water heater and found a three-digit code on the display or remote controller. Maybe the hot water quit first and sent you looking. Either way, you want to know: What does this mean? Is it serious? And what do I do next?

You are in the right place.

This is the most complete guide to A.O. Smith tankless water heater error codes available anywhere — verified against the official manufacturer maintenance documentation for these units, not copied from other websites. We are THE Water Heater Company — licensed water heater specialists with three locations across Southern California, serving Ventura County, Los Angeles County, Orange County, and the City of Santa Barbara. And here is a credential that matters specifically for this article: every single technician on our team is A.O. Smith certified through A.O. Smith University, the manufacturer's own training program. Not one designated "A.O. Smith guy." The whole team.

Two family notes that will save some readers a step:

• If your unit says State, this guide is for you too. State is A.O. Smith's sister brand, and State tankless water heaters share the same platform and the same error codes.
• Here is something most homeowners don't know: A.O. Smith's tankless water heaters are built on the Takagi platform — Takagi, one of the original tankless pioneers in America, is part of the A.O. Smith family of brands. That is why these units use Takagi's proven three-digit code system, and why the official documentation behind this guide is the deepest in the industry.

Before we get to the codes, there is something important you need to understand first.

⚠️ A Critical Safety Warning Before You Read Any Further

A.O. Smith tankless water heaters are not simple appliances. They are sophisticated systems that sit at the intersection of four potentially dangerous forces inside your home:

1. Natural Gas or Propane — A gas leak or improper gas pressure is not something you can safely troubleshoot without professional training and the right tools. Gas leaks can cause fires, explosions, and carbon monoxide poisoning.

2. High-Voltage Electricity — These units use a computer board, thermistors, solenoid valves, and wiring that carry live electrical current. The manufacturer's own diagnostic procedures involve live voltage and resistance testing. Working inside the unit without proper training can cause serious injury or death.

3. Pressurized Water — The water moving through a tankless heater is under pressure. Incorrect handling of connections, valves, or components can cause water damage or scalding burns.

4. Combustion Exhaust — Tankless water heaters produce exhaust gases, including carbon monoxide, that must be properly vented out of your home. A blocked or damaged vent is a life-safety emergency.

This combination of gas, electricity, water, and exhaust is why A.O. Smith tankless water heaters should only be diagnosed and repaired by a licensed, trained professional. This is not a DIY appliance in the same way that changing a light switch might be. The consequences of a mistake are serious.

This guide is written to help you understand what is happening with your unit — not to guide you through a repair. Think of it the same way you would think of a medical reference guide: knowing what a diagnosis means is helpful and important, but it does not replace a trained doctor performing the actual procedure.

If you are in Ventura County, Los Angeles County, Orange County, or the Santa Barbara area and your A.O. Smith or State unit is showing an error code, call THE Water Heater Company at (877) 798-7487. Our phones are answered by a live person 24/7/365, and our A.O. Smith University-certified technicians are here to help.

How Do A.O. Smith Error Codes Work?

A.O. Smith tankless error codes are three-digit numbers (like 111, 121, or 701). Where you see them depends on your setup:

• If you have a remote controller, the code appears right on the display.
• If you do not have a remote controller, the unit signals the error through a blinking green LED on the computer board inside the unit — one very practical reason a technician's visit starts with reading the unit itself.

One behavior worth knowing before you worry: a fan that keeps running for a minute or so after your hot water stops is normal. The manufacturer's documentation confirms the fan intentionally runs after operation to purge exhaust gas from the flue and prepare for quick re-ignition. That sound is your unit finishing its job, not starting a problem.

The Most Important Thing to Understand: Warnings vs. Lockouts

This is something most guides do not explain — and the A.O. Smith/Takagi platform has one of the clearest examples in the industry.

Not all error codes are equal.

Note: A.O. Smith does not use the terms "soft lockout" and "hard lockout" in their publicly available documentation. The following classification is based on THE Water Heater Company's field experience, applied to how each code actually behaves.

What Is a Soft Lockout (Warning)?

A soft lockout means the unit detected a problem, but it may keep operating — for now. The code is your early notice that something is wrong and getting worse.

This platform's escalation pattern is textbook. Per the official maintenance documentation, Code 101 is defined as the warning for Code 991. Code 101 means the unit has detected deteriorating combustion conditions — blocked intake air or exhaust, vent problems, dirt in the burner or fan, dust in the heat exchanger, wrong altitude settings. If those conditions are not corrected, the unit escalates to Code 991 — imperfect combustion — and shuts down.

Your A.O. Smith tells you before it locks you out. Code 101 is the unit saying "fix my air supply while I can still run."

The takeaway: A warning code is a gift. Do not ignore it just because the hot water still works. Call a professional before the warning becomes a lockout.

What Is a Hard Lockout?

A hard lockout means the unit detected a condition where continuing to run would not be safe — failed ignition, flame loss, a failed sensor, a gas valve fault. The unit shuts down and stays down until the underlying problem is found and fixed. Resetting may briefly clear the display, but the code returns because the fault is still there.

Hard lockouts are your A.O. Smith protecting your home. Do not fight the safety system. Call a professional.

Quick Reference: A.O. Smith Tankless Error Codes at a Glance

Code	What It Means (per official platform documentation)	Lockout Type
031	Incorrect DIP switch setting	Configuration fault
101	Combustion warning — early notice for Code 991	Soft (warning — escalates)
111	Ignition failure	Hard
121	Loss of flame	Hard
311	Outlet thermistor disconnected/short-circuited	Hard
321	Inlet thermistor disconnected/short-circuited	Hard
331	Heat exchanger thermistor disconnected/short-circuited	Hard
341	Exhaust thermistor fault (certain indoor models)	Hard
391	Air-fuel ratio rod failure	Hard
441	Flow sensor failure (multi-unit systems)	Hard
510	Abnormal main gas solenoid valve	Hard
551	Abnormal gas solenoid valve	Hard
611	Fan motor fault	Hard
651	Flow adjustment valve fault (multi-unit systems)	Hard
661	Bypass valve fault (certain models)	Hard
701	Computer board failure	Hard
711	Gas solenoid valve drive circuit failure	Hard
721	False flame detection	Hard
741	Remote controller communication fault	Communication fault
751	Temperature controller communication fault (certain indoor models)	Communication fault
761	Multi-unit parent/child communication fault	Communication fault
941	Abnormal exhaust temperature (certain indoor models)	Hard
991	Imperfect combustion	Hard

A.O. Smith and State have produced multiple tankless generations, and some models use additional codes not listed here. If your unit shows a code you don't see above, call us at (877) 798-7487 and we will tell you exactly what it means for your model.

The Codes, One by One

What Does A.O. Smith Error Code 031 Mean?

Code 031 means the DIP switches on the computer board are set incorrectly — the tiny configuration switches that tell the unit its gas type, model type, vent length, and altitude.

Lockout type: Configuration fault.
Common causes: settings disturbed during a board replacement, or a unit never configured correctly at installation.
What you should do: DIP switch configuration is done power-off, against the manufacturer's model-specific tables — and a wrong gas-type setting is a combustion safety issue. Professional adjustment only.

What Does A.O. Smith Error Code 101 Mean?

Code 101 is the combustion warning — officially defined as the early warning for Code 991. Your unit is telling you its combustion conditions are deteriorating while it can still operate.

Lockout type: Soft — a warning that escalates.
Common causes (straight from the official checklist): blockage in the intake air or exhaust, vent terminals too close together on direct-vent installs, excessive vent length, wrong altitude settings, grease or dirt in the burner and fan motor, dust and lint in the heat exchanger, wrong gas type, or incorrect manifold pressure. In our region, garage lint and wildfire-season ash both show up in this story.
What you should do: treat Code 101 as your scheduled-service alarm. A professional combustion service now prevents a 991 shutdown later. ⚠️ Combustion and venting issues are carbon monoxide territory — never a DIY diagnosis.

What Does A.O. Smith Error Code 111 Mean?

Code 111 means ignition failure — the burner tried to light and could not. No ignition, no hot water. This is the most-searched code on the platform.

Lockout type: Hard.
Common causes: gas supply or inlet pressure problems (a partially closed valve, an undersized line, an empty propane tank), a failed igniter, soot on the flame rod, a tripped Hi-limit switch, a failed overheat cutoff fuse, damaged wiring, dust blocking the manifold nozzles, or a leaking heat exchanger. The official diagnostic procedure runs nine steps deep with live voltage testing — which tells you everything about whether this is a DIY code.
What you should do: check the honest basics — is your gas on? Are other gas appliances working? Then call a professional.

A Southern California note: we regularly trace ignition failures on converted homes to gas lines never resized for tankless demand. If your 111 arrived after a bargain conversion, the gas line is our first suspect.

What Does A.O. Smith Error Code 121 Mean?

Code 121 means the flame was lost during operation. The unit lit successfully, then the flame went out — so it shut down for safety.

Lockout type: Hard.
Common causes: marginal gas pressure collapsing under full demand, soot on the flame rod, burn marks on the computer board, a failing Hi-limit switch, a leaking heat exchanger, or blocked manifold nozzles.
What you should do: intermittent flame loss can pass a short test and fail only under sustained demand — professional diagnosis, always.

What Do A.O. Smith Error Codes 311, 321, 331, and 341 Mean?

These are thermistor faults — the unit's temperature sensors reporting as disconnected or short-circuited. Per the official documentation: 311 is the outlet thermistor, 321 the inlet thermistor, 331 the heat exchanger thermistor, and 341 the exhaust thermistor on certain indoor models.

Lockout type: Hard. The control board refuses to guess at temperatures it cannot verify.
Common causes: a failed thermistor, damaged wiring, or debris on the sensor — and in Southern California, mineral scale coating the sensor until it reads the scale instead of the water.
What you should do: thermistor diagnosis is a resistance test against published values, then sensor or board replacement. Professional work — and if scale contributed, ask about descaling while the technician is there, because scale never targets just one component.

What Does A.O. Smith Error Code 391 Mean?

Code 391 means the air-fuel ratio (AFR) rod has failed — the component that helps balance the air-gas mixture feeding the burner.

Lockout type: Hard.
Common causes: soot on the AFR rod or flame rod, or damaged wiring.
What you should do: rod service plus a combustion check — and a good technician finds what caused the soot rather than just cleaning the evidence.

What Does A.O. Smith Error Code 441 Mean?

Code 441 means the flow sensor has failed on multi-unit (linked) systems.

Lockout type: Hard.
Common causes: debris on the flow sensor impeller, damaged wiring, a clogged return filter, or frozen water in the pump piping during cold snaps.
What you should do: professional inspection of the sensor, impeller, and filters.

What Do A.O. Smith Error Codes 510 and 551 Mean?

Codes 510 and 551 mean the main gas solenoid valve or a gas solenoid valve is behaving abnormally — the most safety-critical components in the unit.

Lockout type: Hard. ⚠️ Any code involving the gas valve system is professional-only, full stop.
Common causes: wiring faults, burn marks on the computer board, or a failing valve in the gas valve assembly.
What you should do: the official procedure involves measuring voltage on each valve in the assembly. Call a licensed professional — this is precisely what the Four-Danger warning at the top of this guide exists for.

What Does A.O. Smith Error Code 611 Mean?

Code 611 means the combustion fan motor has a fault. No fan, no safe combustion, no hot water.

Lockout type: Hard.
Common causes (per the official checklist): dust buildup in the fan motor, damaged wiring, burn marks on the board, or corroded fan connectors.
What you should do: professional fan diagnosis. Remember — a fan running for a minute after shutdown is normal purge behavior, not a 611.

What Do A.O. Smith Error Codes 651 and 661 Mean? (The Hard Water Codes)

Code 651 means the flow adjustment valve has failed (multi-unit systems), and Code 661 means the bypass valve has failed (certain models). And here is the sentence that matters for every Southern California homeowner: the manufacturer's own maintenance documentation instructs technicians to inspect these valves for a "locked motor drive due to scale buildup."

The manufacturer's first suspects are scale and water leakage. Scale physically jams the valve's motor until it cannot move.

Lockout type: Hard.
What you should do: professional valve diagnosis — and a serious conversation about scale. If hard water seized a valve, it is also coating your heat exchanger and sensors. From Simi Valley to Santa Clarita to Santa Barbara, annual descaling is not an upsell on this platform; it is how these units reach old age in our water. (Look up your community's water hardness in our water quality library — we maintain profiles for 99 Southern California communities.)

What Does A.O. Smith Error Code 701 Mean?

Code 701 means the computer board — the unit's brain — has failed.

Lockout type: Hard.
Common causes: age, electrical damage (the official guidance says to look for burn marks or brown spots on the board), power surges, or moisture.
What you should do: board diagnosis and replacement is professional work — and on an older unit, this visit becomes an honest repair-versus-replace conversation with real numbers for both paths. We will give you both, straight.

What Does A.O. Smith Error Code 711 Mean?

Code 711 means the gas solenoid valve drive circuit has failed — the electrical circuit controlling the gas valves. (Some websites define 711 differently; the official maintenance documentation is clear, and this guide follows it.)

Lockout type: Hard. ⚠️ Gas valve circuitry — professional-only.
What you should do: the official procedure routes through the same deep diagnostics as ignition failure and flame loss. Call a licensed professional.

What Does A.O. Smith Error Code 721 Mean?

Code 721 means false flame detection — the unit "sees" flame when there should not be one.

Lockout type: Hard.
Common causes (per the official sheet): a dirty flame rod, or water leaking from the heat exchanger interfering with flame sensing.
What you should do: flame rod cleaning and a leak inspection — professional work, since a leaking heat exchanger changes the whole conversation.

What Do A.O. Smith Error Codes 741, 751, and 761 Mean?

These are communication faults: 741 is the water heater and remote controller not talking, 751 is the temperature controller connection on certain indoor models, and 761 is the parent and child units in a multi-unit system losing contact.

Lockout type: Communication fault — heating may continue in some cases, but you have lost control or coordination.
Common causes: wiring connections, an incompatible remote controller model, power supply problems, or a failing board on either end.
What you should do: professional diagnosis — where the code appears (the remote, the board's LED, or both) actually determines the fix, per the official troubleshooting tree.

What Does A.O. Smith Error Code 941 Mean?

Code 941 means abnormal exhaust temperature on certain indoor models — the exhaust is running hotter than it safely should.

Lockout type: Hard. ⚠️ Exhaust codes are safety codes.
Common causes: dust buildup in the fan motor, a failing exhaust thermistor, or board damage.
What you should do: same-day professional service.

What Does A.O. Smith Error Code 991 Mean?

Code 991 means imperfect combustion — the unit has shut down because it cannot burn cleanly. This is the lockout Code 101 warned you about.

Lockout type: Hard. ⚠️ Incomplete combustion is the carbon monoxide code. Take it seriously.
Common causes: everything on the Code 101 list, left uncorrected.
What you should do: do not repeatedly reset a 991 and do not run the unit. Call a professional the same day. And if your home lacks carbon monoxide alarms near sleeping areas, fix that today regardless of your water heater.

Have an A.O. Smith TANK Water Heater With a Blinking Light?

A.O. Smith is America's biggest name in tank water heaters, so plenty of readers arrived here with a tank unit blinking a status light — not a tankless unit showing a three-digit code. Two honest things to know:

1. Tank status lights speak a different language. A.O. Smith gas tank heaters signal their status through LED flash patterns on the gas control valve, and the meaning of those flash patterns varies by gas valve generation and model. A "four flash" pattern on one valve is not the same as on another — which is exactly why we are not printing a generic flash-code chart here. Guides that do are guessing with your gas appliance.
2. The right move is your model number plus a phone call. Read us the model and serial number from the unit's label and describe the flash pattern, and we will tell you what your specific valve is saying — and whether it can wait. Call (877) 798-7487.

Every one of our technicians is A.O. Smith certified through A.O. Smith University — tank and tankless alike. Whichever kind of A.O. Smith is blinking at you, it is speaking a language we were trained on by the manufacturer.

Frequently Asked Questions About A.O. Smith Error Codes

Are State tankless error codes the same as A.O. Smith?
Yes. State is A.O. Smith's sister brand on the same platform — every code in this guide applies.

Why do A.O. Smith and Takagi error codes match?
Because A.O. Smith tankless units are built on the Takagi platform — Takagi is part of the A.O. Smith family. Same engineering, same proven three-digit code system, same official documentation underneath.

How do I reset an A.O. Smith tankless error code?
Power-cycling clears the display on many codes — but if the code returns, the fault is real and resetting is just muting the alarm. Combustion codes (101, 991) and gas valve codes (510, 551, 711) should never be reset-and-forgotten. When in doubt, call (877) 798-7487 and we will tell you over the phone whether your code can wait.

What is the most common A.O. Smith tankless error code?
Ignition failure (111) is the most-searched, typically tracing to gas supply, igniter, or flame rod issues. In Southern California we also see a steady diet of scale-driven faults — the valve codes (651, 661) that the manufacturer's own documentation ties to scale buildup, and thermistor codes on units that have never been descaled.

Is it safe to keep using my A.O. Smith with an error code showing?
A warning code like 101 allows temporary operation — but 101 is officially the pre-shutdown warning for a combustion problem, which is a safety matter. Hard lockout codes shut the unit down for your protection; never force a unit past them.

Does hard water really cause A.O. Smith error codes?
Yes — and on this platform it is not our interpretation, it is in the manufacturer's own troubleshooting sheets, which direct technicians to check the flow adjustment and bypass valves for motor drives locked by scale buildup. Annual descaling and scale prevention are how these units reach old age in our water.

The Honest Truth: Most of These Codes Are Preventable

Look back through this guide and notice the pattern. Dust and lint in the burner, fan, and heat exchanger drive the combustion codes (101, 991, 611). Scale locks up valves (651, 661) and degrades sensors (311–341). Soot on the flame rod feeds the ignition and flame codes (111, 121, 391, 721).

Every one of those is addressed in a single annual maintenance visit: a professional descale of the heat exchanger, filter cleaning, flame rod cleaning, and a full combustion and safety inspection. That is the entire reason annual tankless maintenance exists.

If your A.O. Smith or State unit is in Ventura County, Los Angeles County, Orange County, or the Santa Barbara area, ask us about THE Tankless Protection Plan — annual eco-friendly descale, pre-filter cleaning, flame rod cleaning, a free water test and heat exchanger inspection, priority scheduling, and a $0 diagnostic visit each year. It is how you make this entire article irrelevant to your household.

What Does A.O. Smith Error Code Service Cost?

We believe in transparent pricing — it is written into THE Fair Price Guarantee on every invoice. The honest answer is that error code service depends entirely on what the diagnosis finds: a flame rod cleaning is very different from a gas valve assembly, and a descale is different from a computer board. What we can promise: a flat diagnostic visit with the price told to you up front, a written quote before any work begins, and no surprises on the invoice — ever. Call (877) 798-7487 or build your project online, and you will know your number before we start.

A.O. Smith or State Showing a Code? We're Minutes Away.

THE Water Heater Company services A.O. Smith and State water heaters — tank and tankless — across all of Southern California, with every technician certified through A.O. Smith University:

• Ventura County — 808 Calle Plano, Camarillo, CA 93012 (headquarters)
• Los Angeles County — 13615 Victory Blvd #108, Van Nuys, CA 91401
• Orange County — 630 S Grand Ave #109, Santa Ana, CA 92705

Serving Ventura County, Los Angeles County, Orange County, and the City of Santa Barbara — with same-day service available 7 days a week and phones answered by a live person 24/7/365.

Call (877) 798-7487 — THE Water Heater Company. Your Trusted Water Heater Authority.

Related Reading: More Guides from THE Water Heater Company

• Rinnai Tankless Water Heater Error Codes: The Complete Guide
• Rheem Tankless Water Heater Error Codes: The Complete Guide
• Noritz Tankless Water Heater Error Codes: The Complete Guide
• Navien Tankless Water Heater Error Codes: The Complete Guide
• Bradford White Water Heater Status Light Codes: The Complete Guide
• Takagi Tankless Water Heater Error Codes: The Complete Guide
• Water quality profiles for 99 Southern California communities
• The State of Hot Water 2026 report

Sources

• Takagi (A.O. Smith family), official maintenance sheets 68A166-1, 5M2054, and 62S101-3 (takagi.com) — the authoritative error code documentation for the A.O. Smith/Takagi tankless platform
• Field experience: THE Water Heater Company's A.O. Smith University-certified technicians, Southern California service area