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
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
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More Rinnai tankless water heater codes

02 02 — Power Interruption / Memory Loss 03 03 — Loss of Communication / EZConnect Issue 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 →

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: rinnai.us.