Rinnai · Tankless Water Heater Moderate

34 — Combustion Air Temperature Sensor Fault

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

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

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.

Why it happens

Typical signs: Code 34 on power up or ignition attempt · Burner won't run

  • Faulty combustion air thermistor Sensor failed.
  • Loose harness Connector backed out at PCB.
  • Moisture/corrosion at connector Common on outdoor models.
  • PCB fault Less common.

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.