Takagi Condensing Code 331
Mixing thermistor fault. The sensor on the mixing side — where heated water is blended with bypassed water to hold the outlet temperature — is not reporting correctly.
Part replacementThe verdict
Replace a part
A sensor rather than the valve it watches, which makes it one of the cheaper faults. Expect the outlet temperature to be less stable until it is fixed.
Typical cost: $150–$450
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
- Note whether the temperature swings during a shower.
- Book a service call.
Frequently asked questions
Is this the same as code 661?
No. 331 is the sensor on the mixing side; 661 is the water bypass valve itself. One reports, the other acts.
Code 331 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:
Don't confuse this with
3 other Takagi Condensing codes point at a thermistor too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: