Rinnai tankless water heater code 31
Your series decides which reading applies. Across the 2 Rinnai series documented here, code 31 is listed as burner thermocouple out of range (1 series) and burner sensor fault (1 series). Find your series in the table below, where those readings route to a professional.
What code 31 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Burner thermocouple out of range | Call a pro | $150–$900 |
| RH180 Hybrid | Burner sensor fault | Call a pro | $150–$600 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI and RH180 Hybrid, and each one gets its own reading of code 31 — what Rinnai documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 2 series in four columns, which is enough to route you and not enough to explain anything. This is the Rinnai wording behind each row, one entry per manual, so you can see where the 2 readings of code 31 agree and where they part company.
Non-SENSEI — 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.
RH180 Hybrid — Burner sensor fault
The burner sensor is out of range or not reporting. Rinnai's remedy on the RH180 is a single line — replace the sensor — with none of the wiring and resistance checks it gives for the other sensor codes on this appliance.
How much the answer changes between series
Non-SENSEI and RH180 Hybrid all route code 31 the same way: a professional, at $150–$900. That makes it one of the Rinnai numbers you can act on before working out which unit you own.
The spread is real money. On the RH180 Hybrid the worst case is $600; on the Non-SENSEI the same number can reach $900. That is a $300 gap behind one number.
Which Rinnai do you have?
On the 2 series that report code 31, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 64 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 36), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
Source: Rinnai RH180 Operation and Installation Manual (100000319) (p. 36), read Sep 2026 — the manufacturer's own documentation for this Rinnai range.
The same number on other brands
Code 31 is not a standard across the industry — every manufacturer publishes its own table. If your unit is not a Rinnai, this number means something else entirely:
Parts, guides and symptoms behind code 31
Code 31 on these 2 Rinnai series points at 1 guide, 4 parts and 2 symptoms, each named by the manufacturer's own procedure for this code.