Takagi Condensing Code 391
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.
Call a proThe verdict
Call a licensed plumber
A combustion sensing component inside a sealed gas appliance. The part itself is not the expensive kind, but reaching it means the unit open, and the reason it failed — carbon build-up from poor combustion — matters as much as the part.
Typical cost: $150–$700
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Condensing — not averaged across the brand.
Source: Takagi Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 13), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- 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.
Frequently asked questions
Is the air-fuel ratio rod the same as a flame sensor?
It does a related job. A flame rod answers whether there is flame; an air-fuel ratio rod is used to judge the quality of combustion, which is why Takagi gives it its own code separate from the flame-loss ones.
Code 391 on other Takagi series
Other Takagi series report code 391 under the same fault name, but each manual is transcribed separately and they do not read alike: