Rinnai SENSEI RX Code 34
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.
Part replacementThe verdict
Replace a part
A sensor fault, not a scale emergency — but on this series the sensor in question is the one that detects scale, so a technician will usually clean and re-fit it, check whether the unit is actually scaling up, and only then replace it. An inexpensive part with a call-out around it.
Typical cost: $150–$450
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. 39), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the unit has also been showing LC codes — the two are related, and that pattern matters to the technician.
- Note when it was last flushed and whether your water is hard.
- Book a service call.
Frequently asked questions
I found a page saying code 34 is a combustion air sensor. Which is right?
Both, for different units. Rinnai documents code 34 as the combustion air temperature thermistor on non-SENSEI models and as the lime detecting thermistor on the SENSEI RX/CX series. If your model number starts with RX or RXP, it is the lime sensor.
Does code 34 mean my unit is scaled up?
Not by itself. Code 34 says the lime sensor is not reporting properly. Actual scale build-up shows as LC or the SS service reminder.