Takagi Condensing Code 721
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.
Call a proThe verdict
Call a licensed plumber
The unit refusing to run into a condition it cannot explain is the correct behaviour. The usual finding is the sensing circuit rather than a real flame, but that is a determination for a technician, not an assumption to act on.
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. 13), 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.
- 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.
Frequently asked questions
Could grounding cause a false flame reading?
It is a standard suspicion across brands. Flame sensing passes a small current through the flame to ground, so a poor or reversed ground can make the board read current that is not flame.
Code 721 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: