34 Moderate Moderate DIY

Rinnai Tankless Water Heater 34 — Combustion Air Temperature Sensor Fault

Rinnai Tankless Water Heater · Error code 34

What 34 Means

The combustion air thermistor is open, shorted, or reading out of range.

Symptoms

  • Code 34 on power up or ignition attempt
  • Burner won't run

Common Causes

1
Faulty combustion air thermistor

Sensor failed.

2
Loose harness

Connector backed out at PCB.

3
Moisture/corrosion at connector

Common on outdoor models.

4
PCB fault

Less common.

How to Fix It

  1. Disconnect power.
  2. Locate combustion air thermistor in intake plenum.
  3. Measure resistance and compare to service chart.
  4. Inspect/clean connector and reseat.
  5. Replace thermistor if out of spec.
  6. Replace PCB if sensor is good but code persists.

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.