Rinnai tankless water heater code 34
Your series decides which reading applies. Across the 2 Rinnai series documented here, code 34 is listed as combustion air temperature thermistor fault (1 series) and lime detecting thermistor fault (1 series). Find your series in the table below, where those readings route to a professional and a replacement part.
What code 34 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Combustion air temperature thermistor fault | Call a pro | $150–$900 |
| SENSEI RX | Lime detecting thermistor fault | Part replacement | $150–$450 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI and SENSEI RX, and each one gets its own reading of code 34 — 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 34 agree and where they part company.
Non-SENSEI — Combustion air temperature thermistor fault
This is the clearest example on the whole site of why the series matters. On a Non-SENSEI unit, code 34 is the combustion air temperature thermistor, and it lives in the fan motor housing. On the SENSEI RX it is the lime detecting thermistor clamped to the hot water pipe. They are different parts, in different places, and they do not even read alike: this one is checked on a 20K ohm range at 11.4 to 14K ohms at 59°F, against a 100K ohm range at 70.6 to 85.0K ohms for the RX sensor. Rinnai's list here also runs past the sensor into gas type, gas conversion, combustion pressures and venting.
SENSEI RX — Lime detecting thermistor fault
This is the clearest example on the whole site of why the series matters. On Rinnai's older non-SENSEI units, code 34 is the combustion air temperature thermistor in the fan housing. On the SENSEI RX it is the lime detecting thermistor clamped to the hot water pipe — the sensor that watches for scale building up and drives the LC lockout. It is even a different resistance range: around 70.6-85.0K ohms at 59°F against 11.4-14K ohms for the older sensor.
How much the answer changes between series
This is the part that catches people out. Code 34 does not lead to the same job on every Rinnai series: a professional and a replacement part are both documented answers to this number, depending on the generation. Buying a part because a page for a different series said so is how this fault gets expensive.
The spread is real money. On the SENSEI RX the worst case is $450; on the Non-SENSEI the same number can reach $900. That is a $450 gap behind one number.
Which Rinnai do you have?
On the 2 series that report code 34, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 68 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. 39), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
The same number on other brands
Code 34 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 34
Code 34 on these 2 Rinnai series points at 1 guide, 2 parts and 2 symptoms, each named by the manufacturer's own procedure for this code.