Rinnai Non-SENSEI Code 34
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.
Call a proThe verdict
Call a licensed plumber
A sensor fault on paper, but Rinnai's diagnosis for this generation goes through the gas system — gas type, conversion history, manifold pressures and venting — before it condemns the thermistor. That puts it on the professional side, and it is why a part number pulled from a newer model's page is the wrong part.
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. 39), 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.
- Check whether this unit was ever converted between natural gas and propane. Rinnai asks that question early here, and the answer changes the diagnosis.
- Do not order a part from a SENSEI RX code 34 listing — on your unit this is a different sensor in a different place.
- Book a licensed professional.
Frequently asked questions
Is Rinnai code 34 the lime sensor?
Only on the SENSEI RX and CX generation. On a Non-SENSEI unit the same number is the combustion air temperature thermistor in the fan motor housing — a different component with a resistance range around five times lower. This is the single best reason to check your model number before buying a part for a Rinnai.
Does a SENSEI have a code 34?
No. Rinnai lists code 34 for the Non-SENSEI and the SENSEI RX/CX series only. On the SENSEI generation the number is not assigned.
Code 34 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:
Don't confuse this with
3 other Rinnai Non-SENSEI codes point at a thermistor 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: