Takagi Condensing Code 311
Output thermistor fault. The sensor reading the temperature of water leaving the heater is not reporting correctly. It is the main feedback the unit uses to hold your set temperature.
Part replacementThe verdict
Replace a part
A cheap sensor with a service call around it. Until it is replaced the heater is controlling temperature without its most important reading, so expect it to wander at the tap.
Typical cost: $150–$450
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
- Note how the temperature behaves at the tap.
- Note when the unit was last flushed — scale on a water-side sensor is a common cause.
- Book a service call.
Frequently asked questions
Takagi has four thermistor codes. Which is which?
311 output, 321 inlet, 331 mixing, 341 exhaust. Four sensors in four places, four separate codes — which is unusually specific and makes the diagnosis quicker than on brands that lump them together.
Code 311 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
3 other Takagi Condensing codes point at a thermistor 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: