Rinnai · Tankless Water Heater Moderate

31 — Burner Sensor (Combustion Offset) Fault

Abnormal combustion offset reading. Often a wiring or PCB-side fault on the burner thermistor.

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

How to fix it

  1. Power down and reseat all sensor harnesses at the PCB.
  2. Verify dip-switch settings match installation manual for gas type and model.
  3. Measure burner sensor resistance — compare to spec.
  4. Replace the burner sensor if out of spec.
  5. If sensor and wiring are good, replace the PCB.

Why it happens

Typical signs: Code 31 on power-up or under load · Unit fails to fire or shuts off shortly after firing

  • Faulty burner sensor Thermistor failed or out of spec.
  • Loose wire harness at PCB Connector backed out or corroded.
  • PCB failure Control board can't read sensor.
  • Dip-switch / configuration error Unit set up for wrong gas or model variant.

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.