Takagi Non-Condensing Code 391
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.
Also displayed as: 39
Call a proThe verdict
Call a licensed plumber
A combustion sensing component inside a sealed gas appliance. The part is not the expensive kind, but 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 Non-Condensing — not averaged across the brand.
Source: Takagi Non-Condensing Models On-Demand Water Heater Troubleshooting Guide (p. 12), 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 this the same as a flame sensor?
Related but not the same job. A flame rod answers whether there is flame; an air-fuel ratio rod is used to judge combustion quality, which is why it has its own code separate from the flame-loss ones.
Code 391 on other Takagi series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit: