What do Takagi error codes 111, 121 and 991 mean?

Takagi's guide covers the T-KJr2 (model 110), the T-K4 (model 310) and the T-D2 (model 510), all non-condensing models. The code number depends on the model. The T-D2 uses three digits, and the T-KJr2 and T-K4 show the first two, with a red light that flashes a matching number of times. In the guide:

Here is how Takagi describes the main ones:

• Ignition failure. The heater does not fire after three failed attempts at ignition.
• Flame loss. The flames disappear while the unit is still supposed to be running.
• Abnormal combustion. The guide says to start with the steps for the 101 warning, and to call Takagi's Technical Service Department if nothing works. The manifold gas pressure may need adjustment, and that is a technician task.

Takagi lists nearly the same checks for 111 and 121, in this order:

1. 1The high limit switch. Takagi has the technician confirm the switch has not tripped. It resets with a click, and if the heater then fires, the cause of the trip still has to be found. The guide says to contact Takagi's technical service.
2. 2The gas supply. The heater needs enough gas, and the gas line has to be clear of debris and air. The guide names two culprits: a faulty pressure regulator at the meter, and, on propane, a regulator that gets cold in the colder part of the day.
3. 3Gas pressure. Takagi wants the supply pressure within specification both when the heater is idle and when it is running.
4. 4The clear plastic tube from the proportional gas valve to the combustion chamber. It has to be attached.
5. 5The overheat cutoff fuse. If it has been compromised, the guide says to contact Technical Service.
6. 6The flame sensor. If the flame comes on for only one or two seconds before going out, Takagi says to clean the flame sensor.
7. 7The igniter (111 only). Takagi says a strong blue spark stays steady and in place, and a weak spark jumps around. No spark or a weak spark means checking the wire connections, and then replacing the igniter.
8. 8The vent. Blockages such as bird nests, animals or trash.
9. 9The circuit board or gas solenoid. If those are suspected, Takagi says to contact the Technical Service Department.

Only a couple of those are homeowner checks, and Takagi's guide is written for a technician. It describes opening the cabinet, testing continuity with a meter and even a jumper test on the overheat fuse. None of that is a job to try at home.

### What is abnormal combustion?

Takagi's 101 warning and 991 shutdown describe a heater that is not getting the right mix of air and fuel. The guide's checks are:

• A blockage in the airway, including the venting run, a backflow preventer installed backwards, the heat exchanger and the fan.
• Manifold gas pressure, which a technician can check and, in rare cases, adjust. Takagi's guide warns that adjusting the manifold pressure can cause unexpected combustion conditions and a health hazard, so it should be done only for very strong reasons, such as high elevation, and after consulting Technical Service.
• Contaminated intake air. Dust, lint from a laundry machine nearby, grease from a kitchen or other contaminants call for burner cleaning.
• Weather. High winds or a frozen vent termination flap.
• Altitude. Takagi says installations at about 3,000 feet or higher need changed dipswitches for higher altitude modes. Whatever the altitude, the dipswitch setting has to match the install.
• Dipswitches in general. Incorrect dipswitch settings, especially the gas type, cause the 031 code and a range of operating problems.

### What do the other Takagi codes mean?

The same guide lists a longer set, using the T-D2 numbers first and the two digit T-KJr2 and T-K4 numbers second:

• 311 or 31, output thermistor, and 321 or 32, inlet thermistor. These are the water temperature sensors. Takagi says the resistance should be about 9.0 kilohms at room temperature, about 74 degrees Fahrenheit. The fixes run from checking wire connections to cleaning the sensor probe to replacing it.
• 391 or 39, air-fuel ratio rod failure.
• 510 or 51, abnormal gas valves, and 55, abnormal solenoid gas valve on the two digit models.
• 611 or 61, abnormal fan motor.
• 701 or 70, computer board fault.
• 721 or 72, false flame detection.
• 741 or 74, remote controller communication failure.

Codes like these are sensor and part faults, which a technician tests with a meter before replacing anything.

Put the gas smell rule first. If you smell gas, leave, keep switches and phones inside alone, and call from outside. Southwest Gas takes emergency calls at 877-860-6020, and 911 works for any suspected leak.

With no smell:

1. 1Fuel and valve. Make sure the gas shutoff to the heater is fully open, and, on propane, that the tank is not low.
2. 2Other gas appliances. If the range or dryer is also weak, the supply is the problem, not the heater.
3. 3The vent terminal outside. Clear leaves, a nest or anything against the wall. Never touch a hot exhaust.
4. 4A power reset. Take the heater's power off and on with its power switch. The guide describes a board reset that involves pressing buttons on the computer board, so leave that to a technician.
5. 5The timing. Note whether the code appears on cold mornings on a propane heater. Takagi's guide names that pattern as a regulator clue.

Do not remove the cover. Rinnai's manual, for comparison, says the same about its own flame codes: the steps are for people qualified to service gas systems.

### What will a technician do?

A technician works down Takagi's list. That means checking the supply pressure at idle and under load with a manometer, verifying the manifold pressure, cleaning the flame sensor, testing the igniter spark through the sight glass, inspecting the vent, checking the overheat cutoff fuse for continuity and confirming the dipswitches match the gas type and installation.

The guide notes that the manifold pressure is preset at the factory and is computer controlled, so it should not usually need adjusting.

One caution about scope. This guide is dated 2011 and covers the non-condensing T-KJr2, T-K4 and T-D2. Newer Takagi models can differ. Your rating plate gives the model, and its manual rules over this page.

Call when 111, 121 or 991 returns after a power reset, when several gas appliances are weak, when there is any gas smell, or when the heater has stopped making hot water. HQ Plumbing and Air is licensed and insured and takes emergency calls around the clock. Your estimate is free and the price is upfront.

If your heater is another brand, see our pages on Noritz error codes 11, 90 and 99, Rinnai error code 11 and Navien error code E003. For upkeep in Phoenix's hard water, see our page on how often to descale a tankless water heater. And if the heater is old, our page on whether to repair or replace a water heater can help.

A. O. Smith tankless heaters use the same 111, 121 and 991 numbers. Its own lists are on our pages on A. O. Smith tankless error codes, codes 111 and 121 and codes 101 and 991.