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
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: