Takagi tankless water heater code 121
Flame loss. Code 121 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 121 means on each Takagi series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Condensing | Flame loss | Call a pro | $150–$900 |
| Non-Condensing | Flame loss | 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 121 — 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 121 agree and where they part company.
Condensing — Flame loss
Takagi's own definition: the code appears if the flames disappear while the unit is still supposed to be operating. As with ignition failure, the first check is whether the hi-limit switch has tripped, followed by gas supply and the flame sensing side.
Non-Condensing — Flame loss
The flame disappeared while the unit was still supposed to be running. Shown as 12 on a T-K4 or T-KJr2 remote, or three flashes of the red LED — the same count as ignition failure.
Does it matter which Takagi you have?
Condensing and Non-Condensing all route code 121 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 121, so they are safe to work through before you have identified your exact unit. They also cost nothing.
- If you smell gas, leave and call your gas utility from outside.
- Confirm gas is on and other gas appliances work.
- Book a licensed professional if it repeats.
That is 3 of the 4 checks documented for code 121. 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 121, 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. 15), 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 121
Code 121 on these 2 Takagi series points at 2 parts and 1 symptom, each named by the manufacturer's own procedure for this code.