33 Moderate Moderate DIY

Rinnai Tankless Water Heater 33 — Heat Exchanger Thermistor Fault

Rinnai Tankless Water Heater · Error code 33

What 33 Means

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

Symptoms

  • Code 33 on demand
  • Unit shuts down quickly after firing

Common Causes

1
Failed heat exchanger thermistor

Sensor open/short.

2
Connector loose or corroded

Common in humid mechanical rooms.

3
Wire damage

Harness chafed against exchanger.

4
PCB input issue

Less common.

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.

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.

Common replacement Heat exchanger thermistor View on Amazon→

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 →

Verified against manufacturer documentation: rinnai.us

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.