Rinnai SENSEI RX Code 65
Water flow control valve fault. The water flow servo — the motorised valve that meters flow through the heater — is not doing what the board tells it. Rinnai adds a specific warning for this code: the servo has failed to close during the bath fill function, and the instruction is to turn the water off and stop using bath fill immediately.
Part replacementThe verdict
Replace a part
Stop the bath fill function now — a servo that will not close means water keeps running. Rinnai's own first step is a power cycle to see whether the code clears; if it does not, the servo is replaced, which requires the electrical, gas, and water supplies off and is therefore a professional job.
Typical cost: $200–$600
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. 55), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- If a bath is filling, close the tap and stop the fill from the controller now.
- Switch the heater off and back on at the breaker to see whether the code clears — that is Rinnai's step 2.
- Do not use the bath fill function again until it has been checked.
- If the code returns, book a service call.
Frequently asked questions
Why is a valve that will not close a safety issue?
Because bath fill is an unattended function. Rinnai's instruction to immediately turn off the water flow and discontinue the bath fill controller exists so an unattended tub does not keep filling.