Rinnai tankless water heater code 41
Freeze protection sensor fault. Code 41 means the same thing on all 3 Rinnai series documented here — but the fix, and what it costs, is still worth reading for your exact unit.
What code 41 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Freeze protection sensor fault | Part replacement | $150–$450 |
| SENSEI | Freeze protection sensor fault | Part replacement | $150–$450 |
| SENSEI RX | Freeze protection sensor fault | Part replacement | $150–$450 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI, SENSEI and SENSEI RX, and each one gets its own reading of code 41 — what Rinnai documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 3 series in four columns, which is enough to route you and not enough to explain anything. This is the Rinnai wording behind each row, one entry per manual, so you can see where the 3 readings of code 41 agree and where they part company.
Non-SENSEI — Freeze protection sensor fault
The freeze protection sensor is not reporting correctly. On this generation Rinnai scopes it to external (outdoor) units — the sensor reads outside air, and on indoor Non-SENSEI models it is not in play, which is different from the SENSEI RX/CX where the code applies to every unit. Rinnai also documents that the symptom differs inside the generation: a KB model with the sensor unplugged, cut or shorted will not fire at all, while a VC model will fire and flash code 41 alternating with the set temperature.
SENSEI — 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.
SENSEI RX — 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.
Does it matter which Rinnai you have?
Non-SENSEI, SENSEI and SENSEI RX all route code 41 the same way: a replacement part, at $150–$450. That makes it one of the Rinnai numbers you can act on before working out which unit you own.
Which Rinnai do you have?
On the 3 series that report code 41, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 76 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 44), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
The same number on other brands
Code 41 is not a standard across the industry — every manufacturer publishes its own table. If your unit is not a Rinnai, this number means something else entirely:
Parts, guides and symptoms behind code 41
Code 41 on these 3 Rinnai series points at 1 guide, 1 part and 1 symptom, each named by the manufacturer's own procedure for this code.