Takagi Condensing Code 321
Inlet thermistor fault. The sensor reading incoming cold water temperature is not reporting correctly. It is the reference the unit uses to work out how much heat it has to add.
Part replacementThe verdict
Replace a part
An inexpensive sensor, a service call around it. The unit may keep running while the reading is wrong, which shows up as temperature that is off rather than absent.
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 when the unit was last flushed and whether your water is hard.
- Book a service call.
Frequently asked questions
I have several units linked together. Which one has the fault?
Takagi's own example uses this code: if unit 2 has error 321, unit 2's own display shows 321 and the TM-RE30 remote shows 232 — the first digit being the unit number and the rest the first two digits of the code.
Code 321 on other Takagi series
Other Takagi series report code 321 under the same fault name, but each manual is transcribed separately and they do not read alike:
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: