Rinnai SENSEI Code 05
Bypass flow control device fault. The bypass flow control device — the motorised valve that mixes heated water with water routed around the heat exchanger to hold the outlet temperature steady — is not reading correctly at the PC board. Rinnai checks two resistance pairs at connector D, both expected between 44 and 52 ohms, and then replaces the device.
Part replacementThe verdict
Replace a part
A component fault with a short diagnosis: two resistance readings decide it. The part is moderate, the labour is a call-out with the unit opened and the supplies off. Not a homeowner job, but not one of the expensive ones either.
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 — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 10), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Look for any sign of water inside the cabinet — moisture in a connector produces the same symptom as a failed device, and matters more than the code.
- Book a service call; the readings have to be taken at the PC board.
Frequently asked questions
My newer Rinnai does not have a code 05. Why?
Because Rinnai moved it. On the SENSEI RX/CX generation the bypass flow servo has its own number — code 66 — with a different diagnosis. Code 05 on that generation is not assigned.
What does the bypass valve actually do?
It decides how much water goes through the heat exchanger and how much goes around it. That mix is how the unit holds a steady temperature at the tap when demand changes.
Code 05 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: