Takagi tankless error 99 and warning code 10: abnormal combustion

By Jason Dearmin · Last reviewed November 14, 2026 · How we check facts

What the codes mean

Takagi's table lists code 10 as the "warning for 99 error code" (101 on the T-D2) and code 99 as "abnormal combustion" (991). On the LED both are five flashes. The troubleshooting for 99 says to refer to error code 101 first, then contact Takagi technical service if nothing has worked, because the manifold gas pressure may need adjustment.

What the abnormal combustion checklist says

1. Blockage in the airway: venting length obstruction such as bird nests or trash, a backflow preventer installed backwards, the heat exchanger, or the fan.
2. Manifold pressure: check it and adjust to the correct values if necessary.
3. Dirty intake air: if intake air is contaminated with dust, lint (a laundry machine nearby), grease (kitchens) or other contaminants, clean the burner.
4. Weather related issues: high winds, a frozen vent termination flap and so on.
5. Altitude: installations around or over 3000 feet need dipswitch changes on the computer board for FM+ modes.
6. Make sure all dipswitches are set properly for the type of installation.
7. After the cause is solved, reset the computer board to clear the error code.

Two of those items are winter-specific. High winds and a frozen vent termination flap are named directly in the guide as a cause of code 101 and therefore 99. If your tankless heater vents through a wall in an area that gets freezing rain or drifting snow, the outside termination is the first place to look.

Which parts are for professionals

Measuring and adjusting manifold pressure, cleaning the burner and changing dipswitches are installer tasks, and the guide asks for technical service when nothing else works. A homeowner can check the termination from the ground and make sure nothing is stacked or leaning against it.

Safe homeowner checks for Takagi tankless error 99

1. Look at the outdoor vent terminationFrom the ground, check for snow, ice, a frozen flap, bird nests or trash and clear what you can safely reach.
2. Check clearancesMake sure nothing is stored against or blocking the heater or its vent and intake.
3. Turn off and retry onceClose the hot tap, wait a minute, and try one hot tap again. If the code returns, stop using the heater.
4. Call a technicianGas pressure, burner cleaning and dipswitch settings need a qualified technician.

Frequently asked questions

What is the difference between code 10 and code 99?

Takagi lists code 10 as a warning for the 99 error. Code 99 is the abnormal combustion error itself. Both use five flashes on the T-KJr2 and T-K4.

Can high altitude cause code 99?

Takagi says installations around or over 3000 feet need dipswitch changes on the computer board for FM+ modes, so a wrong setting at altitude can be a cause.

Is it safe to keep using the heater?

Abnormal combustion can involve carbon monoxide. If the code repeats, stop using the heater until a technician checks it, and make sure you have working carbon monoxide alarms.

Sources

• Troubleshooting Guide 321923-000 (9/12/11): Non-Condensing On-Demand Water Heaters, T-KJr2, T-K4 and T-D2 (110, 310 and 510 models) (Takagi (A.O. Smith Water Products Company)) Covers: Takagi T-KJr2, T-K4 and T-D2 non-condensing tankless water heaters, indoor and outdoor. Other Takagi models have their own guides.

Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.