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
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
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: