Rinnai tankless water heater code 33
Heat exchanger thermistor fault. Code 33 means the same thing on all 3 Rinnai series documented here — but the fix, and what it costs, is still worth reading for your exact unit.
What code 33 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Heat exchanger thermistor fault | Part replacement | $150–$450 |
| SENSEI | Heat exchanger thermistor fault | Part replacement | $150–$450 |
| SENSEI RX | Heat exchanger thermistor fault | Part replacement | $150–$450 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI, SENSEI and SENSEI RX, and each one gets its own reading of code 33 — what Rinnai documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 3 series in four columns, which is enough to route you and not enough to explain anything. This is the Rinnai wording behind each row, one entry per manual, so you can see where the 3 readings of code 33 agree and where they part company.
Non-SENSEI — 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.
SENSEI — 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.
SENSEI RX — 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.
Does it matter which Rinnai you have?
Non-SENSEI, SENSEI and SENSEI RX all route code 33 the same way: a replacement part, at $150–$450. That makes it one of the Rinnai numbers you can act on before working out which unit you own.
Which Rinnai do you have?
On the 3 series that report code 33, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 76 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 38), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
The same number on other brands
Code 33 is not a standard across the industry — every manufacturer publishes its own table. If your unit is not a Rinnai, this number means something else entirely:
Parts, guides and symptoms behind code 33
Code 33 on these 3 Rinnai series points at 1 guide, 2 parts and 1 symptom, each named by the manufacturer's own procedure for this code.