Rinnai SENSEI RX Code 66
Bypass flow servo fault. Another code that exists only on the SENSEI RX/CX generation. The bypass flow control device mixes heated and unheated water to hold the outlet temperature steady; code 66 means its resistance readings at the PC board are out of range. Rinnai's first check is whether the harness and connector are wet — and its conclusion is that if the readings are out of range the PC board is at fault, otherwise the bypass flow control device is.
Part replacementThe verdict
Replace a part
A two-way diagnosis with a clear rule: out-of-range readings point at the PC board, in-range readings point at the bypass servo itself. The servo is the cheaper of the two by a wide margin. Either way the unit has to be opened with the supplies off, so this is a service call.
Typical cost: $200–$900
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. 56), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Look for any sign of water getting into the unit — Rinnai's first step on this code is checking whether the harness and connector are wet, which usually means a leak somewhere above.
- Book a service call and mention whether you found moisture.
Frequently asked questions
Does my older Rinnai have a code 66?
No. Rinnai lists code 66 for the SENSEI RX/CX series only. On other generations that number is not assigned.
Why would water be in the connector?
Either a plumbing leak inside the cabinet or condensate going where it should not. Both matter more than the code — water and a gas appliance's electronics do not mix, and it is worth having found before the board fails.