Rinnai SENSEI Code 33
Heat exchanger thermistor fault. The same class of fault as code 32, on the sensor reading the heat exchanger rather than the outgoing water. Rinnai's procedure is identical: check the harness back to the PC board, remove the thermistor and clean any scale off it, then compare its resistance against the published table before replacing it. This generation has no lime detecting thermistor — Rinnai added that on the SENSEI RX as code 34, with a resistance range about five times the standard sensors. On a SENSEI, scale reaches you through this sensor and through the SS warning instead, which is a slower signal.
Part replacementThe verdict
Replace a part
A sensor fault with the same economics as code 32: an inexpensive part and a call-out that needs the water isolated and the unit drained. If the unit is overdue a flush, expect that to be part of the visit.
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 — 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 when the unit was last flushed and whether your water is hard.
- Book a service call — the sensor is not reachable without draining the unit.
Frequently asked questions
Does code 33 mean the heat exchanger is failing?
No — it is the sensor mounted on it. A failing heat exchanger shows up as code 14 or 54, or as visible leaking, not as 33.
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
2 other Rinnai 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: