Rinnai · Tankless Water Heater Moderate

33 — Heat Exchanger Thermistor Fault

The heat exchanger temperature sensor is open, shorted, or out of range.

Keep using it?
Yes, with caution
DIY difficulty
Moderate
Verified against
media.rinnai.us ↗

How to fix it

  1. Power off unit.
  2. Reseat sensor connector and inspect for corrosion.
  3. Measure thermistor resistance vs. service manual chart.
  4. Replace thermistor if reading is out of spec.
  5. If sensor and wiring check OK, replace the PCB.

Why it happens

Typical signs: Code 33 on demand · Unit shuts down quickly after firing

  • Failed heat exchanger thermistor Sensor open/short.
  • Connector loose or corroded Common in humid mechanical rooms.
  • Wire damage Harness chafed against exchanger.
  • PCB input issue Less common.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

  • Heat exchanger thermistor Common replacement
    Amazon →

More Rinnai tankless water heater codes

02 Code 02 — No Burner Operation During Freeze Protection 03 Code 03 — Power Interruption During Bath Fill 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff

All Rinnai codes → All tankless water heater codes →

Frequently asked questions

What does the 33 code mean on a Rinnai tankless water heater?

The heat exchanger temperature sensor is open, shorted, or out of range.

What causes the 33 code on a Rinnai tankless water heater?

Failed heat exchanger thermistor: Sensor open/short. Connector loose or corroded: Common in humid mechanical rooms. Wire damage: Harness chafed against exchanger.

Is it safe to keep using a Rinnai tankless water heater with a 33 error?

Yes, with caution. This is rated a moderate-severity fault.

How do I fix 33 on a Rinnai tankless water heater?

Power off unit. Reseat sensor connector and inspect for corrosion. Measure thermistor resistance vs. service manual chart.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: media.rinnai.us.