Rinnai SENSEI RX Code 41
Freeze protection sensor fault. A SENSEI RX difference worth knowing: on Rinnai's older SENSEI and non-SENSEI units, this sensor exists only on outdoor models, but Rinnai documents it for all SENSEI RX/CX water heaters. If the sensor is open or disconnected, the heater keeps working and simply alternates the display between your set temperature and code 41 — because the sensor's only 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 leave it through a cold snap. With the freeze protection sensor out, the heater cannot start protecting itself when the temperature drops, and Rinnai does not cover freeze damage under warranty. Book the repair before winter, not after.
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 RX — 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 that 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
The heater still works. Can I ignore code 41?
Through summer, arguably. Into winter, no: the sensor is what tells the unit to run its freeze protection, and a burst heat exchanger is not covered by the warranty.
Why does my indoor unit have a freeze protection sensor at all?
Because it is a SENSEI RX. Rinnai lists this sensor on outdoor models only for the older generations, but on all SENSEI RX/CX water heaters.