Rinnai SENSEI RX Code 72
Flame rod fault. The flame rod circuit is not behaving. Rinnai's checks are the wiring back to the PC board, whether the rod is actually touching the flame when the burner fires, carbon build-up on the rod, and its micro-amp output, which should hold at a minimum of 1 microamp. The rod and its gasket are then replaced, and if that does not resolve it, the board.
Call a proThe verdict
Call a licensed plumber
The flame rod is one of the cheapest parts in the unit and one of the least accessible: it lives inside the sealed combustion chamber and its gasket must be replaced whenever it is disturbed. Most of what you pay for is the visit and the micro-amp measurement that tells the technician whether the rod or the board is at fault.
Typical cost: $150–$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. 60), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Turn the unit off and leave it off.
- If you smell gas, leave the building and call your gas utility from outside.
- Book a licensed professional — the flame rod is inside the sealed combustion chamber.
Frequently asked questions
How is code 72 different from code 12?
Code 12 is the outcome — the flame was lost or never proved. Code 72 is the diagnosis pointing at the flame rod circuit itself. In practice they are investigated together.
Rinnai's steps mention checking the heat exchanger. Why, for a flame rod code?
Because a leaking heat exchanger drips onto the burner and disrupts the flame the rod is trying to sense. It is on Rinnai's list for this code, and it is the expensive outcome.