Takagi tankless water heater code 721
False flame detection. Code 721 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 721 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | False flame detection | Call a pro | $150–$900 |
| Non-Condensing | False flame detection | Call a pro | $150–$900 |
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 721 — 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 721 agree and where they part company.
Condensing — False flame detection
The unit is detecting a flame when there should not be one. Either gas is burning when it should not, or the sensing circuit is reporting a flame that is not there — and the heater refuses to start either way.
Non-Condensing — False flame detection
The unit is detecting a flame when there should not be one — either gas is burning when it should not, or the sensing circuit is reporting a flame that is not there. Shown as 72 on a T-K4 or T-KJr2 remote, or six flashes of the red LED.
Does it matter which Takagi you have?
Condensing and Non-Condensing all route code 721 the same way: a professional, at $150–$900. 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 721, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- Turn the heater off and close the manual gas valve below it.
- If you smell gas, leave the building and call your gas utility from outside.
- Book a licensed professional.
That is 3 of the 3 checks documented for code 721. 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 721, 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 721
Code 721 on these 2 Takagi series points at 2 parts, each named by the manufacturer's own procedure for this code.