Takagi tankless water heater code 311
Output thermistor fault. Code 311 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 311 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | Output thermistor fault | Part replacement | $150–$450 |
| Non-Condensing | Output 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 311 — 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 311 agree and where they part company.
Condensing — Output thermistor fault
The sensor reading the temperature of water leaving the heater is not reporting correctly. It is the main feedback the unit uses to hold your set temperature.
Non-Condensing — Output thermistor fault
The sensor reading the temperature of water leaving the heater is not reporting correctly. Shown as 31 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 311 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 311, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- Note how the temperature behaves at the tap.
- Note when the unit was last flushed — scale on a water-side sensor is a common cause.
- Book a service call.
That is 3 of the 3 checks documented for code 311. 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 311, 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 311
Code 311 on these 2 Takagi series points at 1 guide, 1 part and 1 symptom, each named by the manufacturer's own procedure for this code.