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
- media.rinnai.us ↗
How to fix it
- Power down and reseat all sensor harnesses at the PCB.
- Verify dip-switch settings match installation manual for gas type and model.
- Measure burner sensor resistance — compare to spec.
- Replace the burner sensor if out of spec.
- 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
Frequently asked questions
What does the 31 code mean on a Rinnai tankless water heater?
Abnormal combustion offset reading. Often a wiring or PCB-side fault on the burner thermistor.
What causes the 31 code on a Rinnai tankless water heater?
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.
Is it safe to keep using a Rinnai tankless water heater with a 31 error?
Yes, with caution. This is rated a moderate-severity fault.
How do I fix 31 on a Rinnai tankless water heater?
Power down and reseat all sensor harnesses at the PCB. Verify dip-switch settings match installation manual for gas type and model. Measure burner sensor resistance — compare to spec.
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.