Takagi Condensing Code 621
Exhaust fan motor fault (indoor models). An indoor-only code: the exhaust fan motor is not behaving. Outdoor models do not have this fault because they do not have the same vented exhaust arrangement.
Call a proThe verdict
Call a licensed plumber
Exhaust handling on an indoor gas appliance, which is the part of the system that keeps combustion products out of the building. Leave it off and get it looked at.
Typical cost: $250–$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 unit off and leave it off.
- Check that your carbon monoxide alarms work.
- Book a licensed professional.
Frequently asked questions
My unit is outdoors and shows 621.
Takagi marks this code as indoor models only, so that combination is worth reporting as-is when you call — it suggests either a configuration problem or a misread of the display.
Don't confuse this with
One other Takagi Condensing code points at a fan 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: