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
media.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 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 34 code mean on a Rinnai tankless water heater?

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

What causes the 34 code on a Rinnai tankless water heater?

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

Is it safe to keep using a Rinnai tankless water heater with a 34 error?

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

How do I fix 34 on a Rinnai tankless water heater?

Disconnect power. Locate combustion air thermistor in intake plenum. Measure resistance and compare to service 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.