31 Moderate Hard — consider a tech

Rinnai Tankless Water Heater 31 — Burner Sensor (Combustion Offset) Fault

Rinnai Tankless Water Heater · Error code 31

What 31 Means

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

Symptoms

  • Code 31 on power-up or under load
  • Unit fails to fire or shuts off shortly after firing

Common Causes

1
Faulty burner sensor

Thermistor failed or out of spec.

2
Loose wire harness at PCB

Connector backed out or corroded.

3
PCB failure

Control board can't read sensor.

4
Dip-switch / configuration error

Unit set up for wrong gas or model variant.

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.

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.