33
Rinnai Tankless Water Heater
Severity: MinorWhat Does This Error Mean?
Rinnai code 33 points to the outgoing-water thermistor.
The control compares the resistance from this sensor against an expected curve.
If the value is way out of range — open circuit or near-zero — the unit posts code 33 and shuts the burner off.
The sensor is cheap, accessible, and replaceable in 20 minutes by someone comfortable with a multimeter.
Affected Models
- Rinnai RU130
- Rinnai RU160
- Rinnai RU199
- Rinnai V53e
- Rinnai V65e
- Rinnai V75e
Common Causes
- Thermistor failed open from age
- Connector at the control board worked loose
- Wire chafed against the heat exchanger and shorted
- Moisture on the connector pins from a venting condensate leak
- Control board sensor input itself failed (rare)
How to Fix It
-
Power off.
Switch off the dedicated breaker.
Wait 5 minutes for the unit to fully de-energise before opening the cover. -
Find the outgoing thermistor.
Remove the front cover.
The outgoing thermistor sits on the hot-water outlet pipe at the top of the heat exchanger.
It is a small probe with a two-wire cable running to the control board. -
Reseat the connector.
Trace the cable to the board.
Unplug the small connector, blow out any debris, and reseat firmly.
Loose connectors after years of vibration are a frequent cause of code 33 — sometimes that alone fixes it. -
Test resistance at the sensor.
With the sensor cool (room temperature), measure across the two leads with a multimeter.
Rinnai service spec is around 6-12 kilo-ohms at 70°F.
Open circuit (no reading) means a failed sensor.
Near zero ohms means a short. -
Replace the thermistor.
Genuine Rinnai outgoing thermistors are 25-60 USD.
The probe unscrews or unclips depending on model.
Match part number — Rinnai uses several thermistor variants and they are not interchangeable. -
Restore power and test.
Reinstall the cover.
Restore power.
Run hot water at a fixture for 5 minutes — code 33 should be gone and the outlet temperature should match the set point.
Frequently Asked Questions
Will the heater work at all with code 33 active?
No.
Without a trustworthy outlet temperature reading, the control cannot run the burner safely, so it stays off.
You will have cold water until the sensor is repaired or replaced.