Takagi tankless water heater code 991
Abnormal combustion. Code 991 means the same thing on all 2 Takagi series documented here — but the fix, and what it costs, is still worth reading for your exact unit.
What code 991 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | Abnormal combustion | Call a pro | $150–$1,100 |
| Non-Condensing | Abnormal combustion | Call a pro | $150–$1,100 |
Each row opens the full page for that generation. We hold manuals for Condensing and Non-Condensing, and each one gets its own reading of code 991 — what Takagi documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 2 series in four columns, which is enough to route you and not enough to explain anything. This is the Takagi wording behind each row, one entry per manual, so you can see where the 2 readings of code 991 agree and where they part company.
Condensing — Abnormal combustion
The error that code 101 warns about. Takagi's diagnosis for the pair starts with blockage in the airway — vent obstructions, a backflow preventer installed backwards, the heat exchanger, the fan — then manifold pressure, contaminated intake air, weather, and altitude settings.
Non-Condensing — Abnormal combustion
The error that code 101 warns about. Shown as 99 on a T-K4 or T-KJr2 remote, or five flashes of the red LED. The diagnosis is the same list as 101 — airway blockage, manifold pressure, contaminated intake air, weather and altitude settings — but the unit has now stopped.
Does it matter which Takagi you have?
Condensing and Non-Condensing all route code 991 the same way: a professional, at $150–$1,100. That makes it one of the Takagi numbers you can act on before working out which unit you own.
What to check first, whatever your series
These checks are on the manufacturer's list for every Takagi series that reports code 991, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- Stop using the unit and leave it off.
- Check that your carbon monoxide alarms work. If you have none, fit one before it runs again.
- Go outside and check the vent terminal for nests, trash, ice, or a frozen flap.
- Book a licensed professional.
That is 4 of the 4 checks documented for code 991. The rest differ by generation, so if none of those explains it, go to your own series above.
Which Takagi do you have?
On the 2 series that report code 991, the model number on the rating label starts T — match yours against the 3 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Takagi Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 14), read Aug 2026 — the manufacturer's own documentation for this Takagi range.
Source: Takagi Non-Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 12), read Aug 2026 — the manufacturer's own documentation for this Takagi range.
Parts, guides and symptoms behind code 991
Code 991 on these 2 Takagi series points at 3 parts and 2 symptoms, each named by the manufacturer's own procedure for this code.