Rinnai SENSEI RX Code 33
Heat exchanger thermistor fault. The same class of fault as code 32, but 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.
Part replacementThe verdict
Replace a part
A sensor fault, and the same economics as code 32: an inexpensive part, a call-out that needs the water isolated and the unit drained. Scale on the sensor is a real cause, so 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 RX — 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 my heat exchanger is failing?
No — it is about the sensor mounted on it, not the exchanger itself. A failing heat exchanger shows up as codes 14, 54, or visible leaking, not 33.