Rinnai SENSEI Code 41
Freeze protection sensor fault. A difference worth knowing on this generation: Rinnai fits this sensor to **external (outdoor) SENSEI models only**. If it is open or disconnected the heater keeps working and simply alternates the display between your set temperature and code 41 — the sensor's job is to trigger freeze protection, not to run the burner.
Part replacementThe verdict
Replace a part
The unit still makes hot water, so this is not an emergency — but do not carry it into a cold snap. With the sensor out, the heater cannot start protecting itself when the temperature drops, and Rinnai does not cover freeze damage under warranty.
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 SENSEI — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 44), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Check the sensor is not buried under pipe insulation or wrapping — Rinnai's instruction is that it must be open to outside air.
- If the temperature controller has been rewired or moved, have that checked: Rinnai warns a controller connected to the wrong points must be disconnected immediately.
- Book the repair before freezing weather. Freeze damage is not a warranty claim.
Frequently asked questions
My SENSEI is indoors and shows code 41. Is that possible?
It should not be, on this generation — Rinnai lists the freeze protection sensor for external SENSEI models only. If an indoor unit shows it, check whether you actually have a SENSEI RX, where the sensor is fitted to every model.
The heater still works. Can I ignore it?
Through summer, arguably. Into winter, no: a burst heat exchanger is not covered by the warranty.
Code 41 on other Rinnai series
Other Rinnai series report code 41 under the same fault name, but each manual is transcribed separately and they do not read alike:
Don't confuse this with
2 other Rinnai SENSEI codes point at a sensor 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: