Takagi Error Codes: 111, 121, 391, 991 and the Rest

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

Takagi Error Codes: 111, 121, 391, 991 and the Rest

Short answer

Takagi tankless heaters show a three digit code on bigger models and a two digit code or a count of red flashes on smaller ones. 111 is ignition failure, 121 is flame loss, 391 is the air fuel ratio rod, 101 warns that 991, abnormal combustion, is coming, and 721 means flame where there should be none, which is the one to shut the gas off for.

A Takagi tankless water heater tells you what is wrong in one of three ways, depending on the model. Bigger models show a three digit code, smaller ones a two digit code on a remote, or a count of red flashes on the circuit board.

Everything on this page comes from Takagi’s own troubleshooting guide, document 321923-000, for the T-KJr2, T-K4 and T-D2 heaters. It is published by A.O. Smith Water Products, which markets Takagi heaters in North America. Other Takagi models add codes of their own, listed in their manuals.

Before you start

What to carry: a torch and a pen. Ten minutes to read a code.

Gas or electric? These are codes for Takagi gas tankless heaters.

Find your model. On the label on the side of the heater. The same fault shows as 111 on a T-D2, as 11 on a T-KJr2 or T-K4 remote, and as three red flashes on their circuit board.

Two codes where you shut off the gas first. 721, 72, or six red flashes: false flame detection. Takagi’s guide says first and foremost, shut off all gas and power to the unit, because if it is not the flame sensor it is a very dangerous situation.

510 or 51: abnormal gas valves. Takagi says that if the hot water is off but flames can still be seen through the sight glass, shut off the gas and the power immediately.

Takagi’s own note: installation and service must be done by a qualified installer, such as a licensed plumber or gas fitter. The owner’s checks below involve no gas fittings and no wiring.

Every code in Takagi’s table

The same fault three ways, depending on the model. Ignition failure is 111 on a T-D2, 11 on a smaller model's remote, and three red flashes on its board.

T-D2 and remote	T-KJr2, T-K4 remote	Red flashes	What Takagi calls it
031	03	1	Incorrect dipswitch settings
101	10	5	Warning for the 991 code
111	11	3	Ignition failure
121	12	3	Flame loss
311	31	2	Output thermistor
321	32	2	Inlet thermistor
331			Mixing thermistor
391	39	2	Air fuel ratio rod failure
441			Flow sensor, linked systems only
510	51	6	Abnormal gas valves
	55	6	Abnormal solenoid gas valve
611	61	4	Abnormal fan motor
651			Flow adjustment valve, linked systems only
701	70	1	Computer board fault
721	72	6	False flame detection
741	74		Remote controller communication failure
761			Communication failure, linked systems only
991	99	5	Abnormal combustion

The codes an owner can often help

These Takagi codes start with gas, air or dirt. Takagi's own first checks for these involve no tools beyond a torch.

Check 1

Work Takagi's first checks

111, ignition failure. Takagi’s list checks the gas supply: is the gas on, and on propane, is there gas in the tank. It notes that on propane, a cold regulator in the coldest part of the day can cause it. Then it looks for venting blockages such as bird nests, animals or trash.

121, flame loss. The flame lit and went out. Takagi says that if the flame comes on for only a second or two before going out, the flame sensor needs cleaning, which is a job for the installer.

101 and 991, abnormal combustion. 101 is the warning that 991 is coming. Takagi’s first checks are blockages in the airway: the vent, nests and trash, and a backflow preventer fitted backwards. It also names lint from a nearby laundry and grease from a kitchen getting into the intake air.

611, the fan motor. Takagi’s guide says to look for water in the fan housing and on the wiring, and to let anything wet dry completely before the power goes back on.

741, the remote. Takagi says this happens with the wrong model of remote, more than one remote, or a remote unplugged while the power was on.

No code, no hot water. The inlet water filter, and a cross connection between hot and cold, are in Takagi’s list of procedures: what else stops a tankless heater firing.

When it needs a technician

Check 2

Know when to stop

031. The dipswitches on the board are set wrong, and Takagi says the gas type is a common one. Takagi’s fix is to turn the power off and correct the switches, and it belongs with the installer.

311, 321, 331 and 391. Temperature sensors and the air fuel ratio rod. Takagi’s steps involve draining the unit, measuring resistance and cleaning the probes.

510, 55 and 701. Gas valves and the computer board.

721. Shut off first, then Takagi’s technical service.

111 and 121 that return with the gas definitely on. Takagi moves on to gas pressure, the igniter spark and the overheat cutoff fuse. That is technician work.

Keeping the codes away

Check 3

The maintenance Takagi's guide covers

Descaling. Takagi’s maintenance section includes descaling the unit, and hard water makes it a regular job: how a tankless heater is flushed.

The inlet filter. A screen on the cold inlet that catches grit. Cleaning it is in Takagi’s procedures.

Clean intake air. Keep lint, dust and kitchen grease away from the heater’s air intake, which Takagi links to abnormal combustion codes.

A clear vent. Check the vent outside after storms and in spring for nests.

Questions people ask

What does Takagi error 111 mean?

Ignition failure. Takagi’s checks include the gas supply, propane levels, a cold propane regulator in cold weather, and blockages in the vent.

What is error 101?

Takagi lists 101 as a warning for the 991 code, abnormal combustion. It is the heater telling you early that combustion is going wrong, usually from a blocked airway or dirty intake air.

What does 391 mean?

An air fuel ratio rod failure. Takagi’s steps are the wiring, a clear plastic tube between the gas valve and the combustion chamber, and cleaning the rod.

My T-KJr2 has no display. How do I read it?

Count the red LED flashes on the computer board, or fit the optional remote, which shows the two digit code.

Is Takagi part of another company?

Takagi’s troubleshooting guide is published by A.O. Smith Water Products, which markets the heaters in North America.

Can I reset the heater myself?

Takagi’s guide includes resetting the computer board among its procedures. Turning the power off and on often clears a code, and if the cause is still there, it returns.

Sources: Takagi troubleshooting guide 321923-000 for the T-KJr2, T-K4 and T-D2 (A.O. Smith Water Products)