Takagi error 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 series links to the full page: what the manufacturer's manual says for that generation, what you can safely check yourself, and what the repair should cost.
How much the answer changes between series
Every Takagi series that documents code 991 routes it the same way: a professional. That is unusual enough to be worth saying, because on this brand the same number often means different work on different generations. Here it does not — so you can act on this page without first establishing which unit you own.
The cost is documented the same way across all 2 series: $150–$1,100, depending on which of the manufacturer's causes turns out to be yours.
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.
If none of those explains it, go to your own series above — the remaining checks differ by generation, and that is the whole reason this page exists.
Which Takagi do you have?
The model number is on the rating label on the unit itself. Match it against the list below — or 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
Everything the 2 Takagi series point at for this code, in one list. A part named here is one the manufacturer's own procedure reaches for.