Rinnai Non-SENSEI Code 31
Burner thermocouple out of range. The burner thermocouple is reading outside the range Rinnai expects. Normal combustion gives 20 to 27 millivolts; the unit sets a code 31 above 30 mV or below 8 mV. The direction matters. A high reading suggests the unit is over-firing, or that blocked heat exchanger fins are trapping heat. A low reading suggests low gas pressure or low-BTU gas — the thermocouple is not seeing a hot enough chamber.
Call a proThe verdict
Call a licensed plumber
Diagnosed with a meter on a live burner, and the answer is either manifold gas pressure or a blocked heat exchanger. Both are gas-side professional work; neither has a homeowner version.
Typical cost: $150–$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 Non-SENSEI — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 36), 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.
- Note when the unit was last flushed — a blocked heat exchanger is one of the two directions this fault runs in, and it is the one you can influence.
- If you smell gas, leave and call your gas utility from outside.
- Book a licensed professional. The reading has to be taken while the burner is firing.
Frequently asked questions
Does a newer Rinnai have a code 31?
No. Rinnai assigns code 31 to the Non-SENSEI generation and to the RH180 hybrid only. The SENSEI and SENSEI RX generations do not use a burner thermocouple in this way and have no code 31.
Code 31 on other Rinnai series
Code 31 is a different fault on every other Rinnai series that documents it, which is exactly why this page is written for the Non-SENSEI alone:
Don't confuse this with
3 other Rinnai Non-SENSEI codes point at the burner too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: