Takagi tankless water heater error codes
23 codes that Takagi publishes for its tankless water heaters, each with what it means, the likely cause, what you can safely check and when to call a licensed professional. Tap your code:
If you smell gas
Stop: do not reset, relight or check anything on this page. Takagi’s own words, from the Owner’s Guide operating label, page 42, under “What to do if you smell gas”:
- Do not try to light any appliance.
- Do not touch any electric switch; do not use any phone in your building.
- Immediately call your gas supplier from a neighbor’s phone. Follow the gas supplier’s instructions.
- If you cannot reach your gas supplier, call the fire department.
Calling from a neighbor’s phone means leaving the building first. Source: On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone. Everything else is for a licensed professional — in the manuals’ words, a qualified installer, service agency or the gas supplier. If you smell gas, do none of the checks: follow the maker’s gas-smell instructions at the top of the page. Codes that need one are marked Licensed pro.
Read this first
Takagi tankless water heaters show a three-digit error code on the temperature controller or remote controller (111, 121, 991 …), and a green LED on the circuit board blinks a count at the same time (three blinks for 111). This page follows the error-code table in the Installation Manual and Owner’s Guide of the condensing T-H3 family (240 T-H3J, 340 T-H3S, 540 T-H3). Takagi’s table lists diagnosis steps meant for whoever services the unit; this page names the few an owner can do from outside and marks the rest for a licensed professional. On a 540 in an Easy-Link system the controller alternates the code with the number of the unit that has the fault.
111 Ignition failure
- What it means
- The heater could not light its burner. Green LED: three blinks.
- Likely cause
- Takagi’s checks are the hi-limit switch, broken wires, burn marks on the circuit board, soot on the flame rod, the spark, the gas valve’s “double clunk”, and a leaking heat exchanger. A closed gas valve stops ignition too: Takagi’s operating instructions say to open the manual gas valve on the outside of the unit by hand.
- What you can safely check
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone.
- Make sure the manual gas valve on the outside of the unit is open (counterclockwise to ON, by hand only).
- When to call a licensed professional
- If 111 stays with gas supplied, call a licensed professional: the rest of Takagi’s checks are inside the unit.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
121 Loss of flame
- What it means
- The flame went out while the heater was running. Green LED: three blinks.
- Likely cause
- Takagi’s checks: the hi-limit switch, broken wires, burn marks on the circuit board, soot on the flame rod, a leaking heat exchanger.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
101 Warning before 991 (combustion)
- What it means
- An early warning that combustion is going wrong — if it is not fixed the heater goes on to 991. Green LED: five blinks.
- Likely cause
- Wrong gas type, blocked air intake or exhaust, intake and exhaust terminals too close on a direct-vent system, high altitude, or grease and dirt in the burner and fan (heaters installed in dusty, greasy or lint-filled areas).
- What you can safely check
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone.
- Look at the air-intake and exhaust vent ends outside and clear anything blocking them.
- When to call a licensed professional
- Call a licensed professional for the gas type, vent distances, altitude setting and cleaning the burner and fan.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
991 Imperfect combustion
- What it means
- The heater has detected poor combustion and stops. Green LED: five blinks.
- Likely cause
- The same causes as 101: wrong gas type, blocked intake or exhaust, terminals too close, altitude, or grease and dirt in the burner and fan. Takagi also asks how long the unit has been installed and what the surroundings are like.
- What you can safely check
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone.
- Look at the air-intake and exhaust vent ends outside and clear anything blocking them.
- When to call a licensed professional
- Call a licensed professional: gas type, altitude, vent layout and cleaning the burner are for them.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
941 Abnormal exhaust temperature
- What it means
- Indoor models only: the exhaust is too hot. Green LED: five blinks.
- Likely cause
- Takagi’s check is a set temperature above 140°F (60°C) on a recirculation system.
- What you can safely check
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone.
- If your system recirculates hot water, check whether the temperature is set above 140°F (60°C) on the controller.
- When to call a licensed professional
- Call a licensed professional if 941 stays at a lower setting.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
031 Incorrect DIP switch setting
- What it means
- The configuration switches on the circuit board are set wrong. Green LED: one blink.
- Likely cause
- DIP switches on the circuit board (part 701) set incorrectly, often after installation or a repair.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
311 321 331 341 Temperature sensor (thermistor) failure
- What it means
- 311: outlet thermistor (240 and 340) or heat exchanger thermistor (540). 321: inlet thermistor. 331: outlet thermistor (540 only). 341: exhaust thermistor (indoor models). Green LED: two blinks.
- Likely cause
- Broken wires or debris on the thermistor.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
391 Air-fuel ratio rod failure
- What it means
- The rod that monitors the air-fuel ratio reports a fault. Green LED: two blinks.
- Likely cause
- Broken wires or soot on the AFR rod.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
510 551 Gas solenoid valve fault
- What it means
- 510: abnormal main gas solenoid valve. 551: abnormal gas solenoid valve. Green LED: six blinks.
- Likely cause
- Broken wires or burn marks on the circuit board.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
611 Fan motor fault
- What it means
- The combustion fan is not working. Green LED: four blinks.
- Likely cause
- Broken wires, dust in the fan motor, burn marks on the circuit board, or frozen or corroded connectors.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
651 661 Water valve fault
- What it means
- 651: flow adjustment valve fault (Easy-Link systems only). 661: bypass valve fault (540 only). Green LED: four blinks.
- Likely cause
- Broken wires, a motor locked by scale, or a water leak at the valve.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
441 Flow sensor failure (Easy-Link)
- What it means
- Easy-Link systems only: a flow sensor failure. Green LED: two blinks.
- Likely cause
- Broken wires or debris on the flow sensor impeller.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
701 711 Circuit board fault
- What it means
- 701: computer board fault. 711: gas solenoid valve drive circuit failure — Takagi says to check it as for 111 and 121. Green LED: one blink.
- Likely cause
- Broken wires to the board, or the valve drive circuit.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
721 False flame detection
- What it means
- The heater senses a flame when there should be none. Green LED: six blinks.
- Likely cause
- On indoor models, a missing condensate drain on the vent collar; or a leaking heat exchanger.
- What you can safely check
Nothing for the owner to check: this one is for a licensed professional.
- When to call a licensed professional
- Licensed pro Call a licensed professional or Takagi (1-877-737-2840): every check Takagi lists for this code is inside the unit, and its table ends “after checking, consult with the manufacturer”.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
741 751 761 Communication fault
- What it means
- 741: the water heater and the remote controller are not communicating. 751: the water heater and the temperature controller (indoor models). 761: the units of an Easy-Link system. No green LED count.
- Likely cause
- The wrong model of remote controller, loose connections between the heater and the controller or between linked units, or the heater’s power supply.
- What you can safely check
Tankless gas water heater: do only the owner checks its manual gives for that code — open the gas and water shut-off valves, check a propane tank has gas, clean a filter the way the manual shows you, clear snow or debris from the vent ends outside, turn it off and on at the controller. Leave gas pressure, the burner, the gas valve, wiring and circuit boards alone.
- Check that the water heater has power.
- When to call a licensed professional
- Call a licensed professional to check the controller model and the connections.
Source: Takagi, On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
A code that is not listed here
Older Takagi models use shorter codes for the same faults: Takagi’s troubleshooting guide for the non-condensing T-KJr2, T-K4 and T-D2 shows 11 for ignition failure and 12 for flame loss on some controllers, and only a count of red-LED flashes on the circuit board. Check your model’s manual if your code has two digits.
We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.
Related pages
Source
Every code on this page comes from these pages, published by Takagi and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.
- Takagi: On-Demand Water Heater Installation Manual and Owner’s Guide, 240 (T-H3J), 340 (T-H3S), 540 (T-H3) (68A161-2), Error codes, pages 52–54 Checked 2026-09-29.
- Takagi: Non-Condensing Models Troubleshooting Guide, 110 (T-KJr2), 310 (T-K4), 510 (T-D2) (321923), table 2 Error Codes Checked 2026-09-29.