Takagi Condensing Code 121
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.
Call a proThe verdict
Call a licensed plumber
One occurrence after a gas interruption can be nothing. A repeating code 121 means the flame is genuinely going out or not being seen, and the diagnosis needs gas pressure measured under load.
Typical cost: $150–$900
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. 15), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- If you smell gas, leave and call your gas utility from outside.
- Confirm gas is on and other gas appliances work.
- Note whether it happens when another gas appliance fires — that points at gas supply sizing.
- Book a licensed professional if it repeats.
Frequently asked questions
How is 121 different from 111?
Code 111 is three failed attempts to light. Code 121 is losing the flame after it was established. They share most of their causes, which is why Takagi's diagnostics for the two overlap.
Code 121 on other Takagi series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
One other Takagi Condensing code points at flame detection 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: