Takagi Tankless Water Heater Error Codes: The Complete Guide

By the Certified Technicians at THE Water Heater Company
Licensed Water Heater Specialists | Three Locations Serving Ventura County, Los Angeles County, Orange County & Santa Barbara Since 2018
Last Updated: July 2026

Your Takagi Is Talking to You. Here's How to Understand What It's Saying.

You checked your Takagi tankless water heater and found a three-digit code on the remote controller — or a blinking green light on the unit itself. Maybe the hot water quit mid-shower. 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 Takagi tankless water heater error codes available anywhere — written directly from Takagi's own official maintenance documentation, not copied from other websites (several of which, we found while researching this guide, define some codes incorrectly). 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. Our technicians service Takagi units regularly from Camarillo to Van Nuys to Santa Ana, and every one of our technicians is factory-certified through A.O. Smith University — relevant here because Takagi is part of the A.O. Smith family of brands.

Takagi earned its reputation honestly: it was one of the original tankless pioneers in the American market, and there are a lot of hard-working Takagi units in Southern California garages. When yours shows a code, here is what it is telling you.

But first, something important.

⚠️ A Critical Safety Warning Before You Read Any Further

Takagi 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 — Takagi units use a computer board, thermistors, solenoid valves, and wiring that carry live electrical current. Takagi's own diagnostic procedures involve live voltage 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 — and as you will see below, Takagi's combustion codes take venting very seriously.

This combination of gas, electricity, water, and exhaust is why Takagi 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 Takagi 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 we are here to help.

How Do Takagi Error Codes Work?

Takagi 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 (Takagi's TM-RE40 is the standard), 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 — which is one very practical reason a technician's visit starts with reading the unit itself.

One quirk worth knowing: a fan that keeps running for a little over a minute after your hot water shuts off is not an error. Takagi's documentation confirms the fan intentionally runs 15 to 70 seconds after operation to purge exhaust gas from the flue and prepare for quick re-ignition. That sound is your unit doing its job.

The Most Important Thing to Understand: Warnings vs. Lockouts

This is something most guides do not explain — and Takagi has one of the clearest examples of it in the industry.

Not all error codes are equal.

Note: Takagi 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.

Takagi's own escalation pattern is the perfect example. Per Takagi's official maintenance documentation, Code 101 is defined as the warning for Code 991. Code 101 means the unit has detected imperfect combustion conditions — blocked intake air or exhaust, vent problems, dirt and grease 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.

In other words: your Takagi tells you before it locks you out. Code 101 is the unit saying "fix my air supply while I can still run." Ignoring it has a scheduled ending.

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 — usually at the worst possible moment.

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 fried 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 immediately because the fault is still there.

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

Quick Reference: Takagi Error Codes at a Glance

Code	What It Means (per Takagi's official 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 (indoor T-H3/540 models)	Hard
391	Air-fuel ratio rod failure	Hard
441	Flow sensor failure (Easy-Link 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 (Easy-Link systems)	Hard
661	Bypass valve fault (T-H3/540 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 (indoor T-H3/540)	Communication fault
761	Easy-Link parent/child unit communication fault	Communication fault
941	Abnormal exhaust temperature (indoor T-H3/540 models)	Hard
991	Imperfect combustion	Hard

Takagi has produced many models over more than two decades — the T-K series, T-H series, T-M series, 540 series, and today's A.O. Smith-era models — 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 Takagi Error Code 031 Mean?

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

Lockout type: Configuration fault.
Common causes: settings disturbed during a board replacement or service visit, or a unit that was never configured correctly at installation — including the high-altitude setting, which matters in our mountain communities.
What you should do: DIP switch configuration is done with the power off, against Takagi's model-specific tables. This is a professional adjustment — a wrong gas-type setting is a combustion safety issue, not a preference.

What Does Takagi Error Code 101 Mean?

Takagi Code 101 is the combustion warning — Takagi's official documentation defines it as the warning for Code 991 (imperfect combustion). Your unit is telling you its combustion conditions are deteriorating while it can still operate.

Lockout type: Soft — a warning that escalates.
Common causes (all straight from Takagi's official checklist): blockage in the intake air or exhaust, vent terminals too close together on direct-vent installs, excessive vent length, wrong altitude/DIP settings, grease or dirt in the burner and fan motor (especially in contaminated locations — think garage dust, lint, and, in our region, wildfire-season ash), dust and lint in the heat exchanger, wrong gas type, or incorrect manifold pressure.
What you should do: treat Code 101 as your scheduled-service alarm. The unit is measurably struggling to breathe. A professional combustion service now prevents a 991 shutdown later. ⚠️ Combustion and venting issues are carbon monoxide territory — never a DIY diagnosis.

What Does Takagi Error Code 111 Mean?

Takagi Code 111 means ignition failure — the burner tried to light and could not. No ignition, no hot water. This is the single most-searched Takagi code, and one of the most common we see.

Lockout type: Hard.
Common causes: gas supply or inlet gas 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, damaged wiring, a failed overheat cutoff fuse, dust and lint blocking the manifold nozzles, or a leaking heat exchanger. Takagi's official diagnostic procedure for this code runs nine steps deep, including live voltage tests on the igniter and gas valve assembly — 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. Diagnosing 111 properly means gas pressure measurement and live electrical testing.

A Southern California note: we regularly trace ignition failures on converted homes to gas lines that were never resized for tankless demand. A tankless unit at full fire needs far more gas than the tank it replaced. If your 111 showed up after a bargain conversion, the gas line is our first suspect.

What Does Takagi Error Code 121 Mean?

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

Lockout type: Hard.
Common causes: marginal gas pressure that collapses when the unit ramps to full fire, 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 is genuinely tricky to diagnose — it can behave perfectly during a short test and fail only under sustained demand. Professional diagnosis, always.

What Do Takagi Error Codes 311, 321, 331, and 341 Mean?

These are thermistor faults — Takagi's temperature sensors reporting as disconnected or short-circuited. Per Takagi's official documentation: 311 is the outlet thermistor (water leaving the unit), 321 is the inlet thermistor (water entering), 331 is the heat exchanger thermistor, and 341 is the exhaust thermistor on indoor T-H3/540 models. Some models also use 351 for an additional thermistor.

Lockout type: Hard. The control board refuses to guess at temperatures it cannot verify.
Common causes: a failed thermistor, damaged or disconnected wiring, or debris on the sensor. In Southern California, mineral scale coating a thermistor is a classic way these sensors drift and die — the sensor ends up reading the scale, not the water.
What you should do: thermistor diagnosis is a resistance test against Takagi's published values, then replacement of either the sensor or the board. Professional work — and if scale contributed, your heat exchanger is next on scale's list. Ask about descaling while the technician is there.

What Does Takagi Error Code 391 Mean?

Takagi Code 391 means the air-fuel ratio (AFR) rod has failed. The AFR rod helps the unit balance the mixture of air and gas feeding the burner — it is part of how a Takagi keeps combustion clean and efficient.

Lockout type: Hard.
Common causes: soot buildup on the AFR rod or flame rod, or damaged wiring.
What you should do: rod cleaning or replacement plus a combustion check. Worth noting: soot on the rods is a symptom — something caused incomplete combustion in the first place, and a good technician finds that cause instead of just cleaning the evidence.

What Does Takagi Error Code 441 Mean?

Takagi Code 441 means the flow sensor has failed on Easy-Link multi-unit systems — the sensor that measures how much water is moving through the unit.

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

What Do Takagi Error Codes 510 and 551 Mean?

Takagi Codes 510 and 551 mean the main gas solenoid valve or a gas solenoid valve is behaving abnormally. These are the valves that admit gas to your burner — 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: Takagi's diagnostic procedure involves measuring voltage on each valve in the assembly. Call a licensed professional — this is precisely the kind of code the Four-Danger warning at the top of this guide exists for.

What Does Takagi Error Code 611 Mean?

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

Lockout type: Hard.
Common causes (per Takagi's official checklist): dust buildup in the fan motor, damaged wiring, burn marks on the computer board, or corroded/frozen fan connectors.
What you should do: professional fan diagnosis and replacement. Remember — a fan that runs for up to 70 seconds after the unit stops is normal purge behavior, not a 611.

What Do Takagi Error Codes 651 and 661 Mean? (The Hard Water Codes)

Code 651 means the flow adjustment valve has failed (Easy-Link systems), and Code 661 means the bypass valve has failed (T-H3/540 models). These are motorized valves that regulate water movement inside the unit — and here is the part that matters for every Southern California homeowner: Takagi's own official documentation instructs technicians to inspect these valves for a "locked motor drive due to scale buildup."

Read that again. The manufacturer's first suspects for these failures are scale and water leakage. Scale physically jams the valve's motor drive until it can no longer move.

Lockout type: Hard.
What you should do: professional valve diagnosis and replacement — and a serious conversation about scale. If hard water seized a valve, it is also coating your heat exchanger and sensors. In our service area — from Simi Valley to Santa Clarita to Santa Barbara, some of the hardest water in California — annual descaling is not an upsell on a Takagi. It is how you keep codes 651, 661, and their thermistor cousins off your display. (You can look up your community's water hardness in our water quality library — we maintain profiles for 99 Southern California communities.)

What Does Takagi Error Code 701 Mean?

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

Lockout type: Hard.
Common causes: age, electrical damage (look for burn marks or brown spots, per Takagi's own guidance), power surges, or moisture.
What you should do: board diagnosis and replacement is professional work. On older Takagi models, this visit often becomes an honest repair-versus-replace conversation — a board on a 15-year-old pioneer-era unit deserves a real cost-benefit talk, and we will give you both numbers straight.

What Does Takagi Error Code 711 Mean?

Takagi Code 711 means the gas solenoid valve drive circuit has failed — the electrical circuit that controls the gas valves. (You may find websites defining 711 differently; Takagi's official maintenance documentation is clear, and this guide follows it.)

Lockout type: Hard. ⚠️ Gas valve circuitry — professional-only.
What you should do: Takagi's own procedure for 711 routes through the same deep diagnostics as ignition failure and flame loss. Call a licensed professional.

What Does Takagi Error Code 721 Mean?

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

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

What Do Takagi 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 (indoor T-H3/540 models), and 761 is the parent and child units in an Easy-Link 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, a wrong remote controller model (Takagi's documentation specifies the TM-RE40 as the correct remote), power supply problems, or a failing board on either end.
What you should do: professional diagnosis — the fix ranges from a connection repair to a controller or board replacement, and the right answer depends on where the code appears (the remote, the board's LED, or both).

What Does Takagi Error Code 941 Mean?

Takagi Code 941 means abnormal exhaust temperature on indoor T-H3/540 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. Hot exhaust has a cause, and every possible cause matters.

What Does Takagi Error Code 991 Mean?

Takagi Code 991 means imperfect combustion — the unit has shut down because it cannot burn cleanly. This is the lockout that 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 — blocked intake or exhaust, vent problems, contaminated burner and fan, wrong altitude or gas settings.
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 does not have carbon monoxide alarms near sleeping areas, fix that today regardless of your water heater.

Frequently Asked Questions About Takagi Error Codes

How do I reset a Takagi error code?
Power-cycling the unit 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.

Where do I see the error code if I don't have a remote controller?
The unit signals errors through a blinking green LED on the computer board inside the cabinet. Counting LED flashes accurately (and safely) is genuinely easier for a technician — and reading the code is only step one of the diagnosis anyway.

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

Is it safe to keep using my Takagi with an error code showing?
If the unit is still running with a warning code like 101, it will operate temporarily — but 101 is officially the pre-shutdown warning, and the underlying combustion problem is a safety matter, not a comfort matter. Hard lockout codes shut the unit down for your protection; never force a unit past them.

My Takagi is quite old. Should I repair it or replace it?
Takagi was a tankless pioneer, so Southern California has many units well past 15 years old. Our honest rule: we quote you the repair and, when age makes it relevant, the replacement — with real numbers for both — and you decide. On a newer unit, most codes are absolutely worth repairing.

Does hard water really cause Takagi error codes?
Yes — and on a Takagi this is not our interpretation, it is in the manufacturer's own troubleshooting sheets, which direct technicians to check the flow adjustment valve and bypass valve for motor drives locked by scale buildup. Annual descaling and scale prevention are how Takagi 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 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, pre-filter and inlet filter cleaning, flame rod cleaning, and a full combustion and safety inspection. That is the entire reason annual tankless maintenance exists — and on a Takagi in Southern California water, it is the difference between a code-free unit and a frequent flyer.

If your Takagi 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 Takagi 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.

Takagi Showing a Code? We're Minutes Away.

THE Water Heater Company services Takagi tankless water heaters across all of Southern California:

• 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. Every technician on our team is factory-certified through A.O. Smith University, the training arm of Takagi's parent company.

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
• A.O. Smith 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), T-H3 Maintenance Sheet 68A166-1 — official error code and diagnostic documentation (takagi.com)
• Takagi (A.O. Smith), Maintenance Sheet 5M2054 — official error code documentation (takagi.com)
• Takagi (A.O. Smith), T-D2 Maintenance Sheet 62S101-3 — official error code documentation (takagi.com)
• Field experience: THE Water Heater Company certified technicians, Southern California service area