Takagi tankless water heater code 321
Inlet thermistor fault. Code 321 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 321 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | Inlet thermistor fault | Part replacement | $150–$450 |
| Non-Condensing | Inlet thermistor fault | Part replacement | $150–$450 |
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 321 — 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 321 agree and where they part company.
Condensing — 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.
Non-Condensing — Inlet thermistor fault
The sensor reading incoming cold water temperature is not reporting correctly — the reference the unit uses to decide how much heat to add. Shown as 32 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 321 the same way: a replacement part, at $150–$450. 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 321, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- Note when the unit was last flushed and whether your water is hard.
- Book a service call.
That is 2 of the 2 checks documented for code 321. 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 321, 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 321
Code 321 on these 2 Takagi series points at 1 part and 1 symptom, each named by the manufacturer's own procedure for this code.