Takagi Condensing Code 551
Solenoid valve fault. The solenoid valves in the gas train are not behaving. Solenoids are the electrically operated gates that open to admit gas and close to stop it — the components that make a gas valve a safety device rather than a tap.
Call a proThe verdict
Call a licensed plumber
A gas valve fault, professional only. Because solenoids are what close the gas off, this is not a code to run past while waiting for an appointment.
Typical cost: $250–$1,100
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
Should I close the gas valve myself?
Closing the manual shut-off valve below the unit — the one with a handle you turn a quarter turn — is a reasonable thing for a homeowner to do and is not gas work. Anything beyond that is not.
Don't confuse this with
4 other Takagi Condensing codes point at a valve 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: