Rinnai Non-SENSEI Code 33
Heat exchanger thermistor fault. The sensor reading heat exchanger temperature is out of range. Same procedure as code 32 and the same resistance curve; what differs is where the sensor sits and therefore what a wrong reading lets the unit do. Rinnai's step 2 is to clean scale build-up off the sensor before condemning it.
Part replacementThe verdict
Replace a part
An inexpensive sensor with a service call around it. If it keeps coming back after replacement, the honest read is that the unit is scaling faster than it is being flushed.
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 Non-SENSEI — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 38), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note the unit's flushing history — this sensor is one of the first places scale shows up.
- Book a service call; the unit must be isolated and drained to remove the sensor.
Code 33 on other Rinnai series
Other Rinnai series report code 33 under the same fault name, but each manual is transcribed separately and they do not read alike:
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: