Takagi tankless water heater code 391
Air-fuel ratio rod failure. Code 391 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 391 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | Air-fuel ratio rod failure | Call a pro | $150–$700 |
| Non-Condensing | Air-fuel ratio rod failure | Call a pro | $150–$700 |
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 391 — 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 391 agree and where they part company.
Condensing — Air-fuel ratio rod failure
The air-fuel ratio rod is the sensing element Takagi uses to judge how well the burner is running — not just whether there is a flame, but whether the mixture is right. This code says it has failed.
Non-Condensing — Air-fuel ratio rod failure
The air-fuel ratio rod — the sensing element Takagi uses to judge how well the burner is running, not just whether there is a flame — has failed. Shown as 39 on a T-K4 or T-KJr2 remote, or two flashes of the red LED.
Does it matter which Takagi you have?
Condensing and Non-Condensing all route code 391 the same way: a professional, at $150–$700. 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 391, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- Turn the unit off and leave it off.
- If you smell gas, leave the building and call your gas utility from outside.
- Book a licensed professional.
That is 3 of the 3 checks documented for code 391. 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 391, 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. 13), 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 391
Code 391 on these 2 Takagi series points at 1 part, each named by the manufacturer's own procedure for this code.