Rinnai Error Code 31: Inlet Thermistor Failure
Warning: Your Rinnai water heater is locked out and needs professional attention to prevent water supply interruption.
How to Fix Rinnai Code 31
- Check if the water heater is receiving power. If not, reset the GFCI outlet or replace the fuse.
- If the water heater has power, do not attempt to fix the issue yourself. Contact a professional HVAC technician.
How to Reset Rinnai Code 31
- Turn off the unit using the power button on the front panel.
- Turn off the gas supply valve.
- Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
- Restore power at the breaker, then open the gas valve.
- Turn the unit back on. If the error returns, the underlying issue needs repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Rinnai Error Code 31 Mean?
Rinnai's error code 31, Inlet Thermistor Failure, indicates a problem with the sensor that measures the temperature of the incoming water. This sensor helps regulate the water heater's operation and ensure safety. If it malfunctions, the water heater will shut down to prevent overheating or freezing, displaying error code 31 on the digital display.
Common Causes
- Faulty inlet thermistor
- Wiring issue between the thermistor and the control board
- Control board malfunction
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| Water heater is not heating water | Measure the resistance of the inlet thermistor using a multimeter | Between 10,000 ohms and 100,000 ohms, depending on the water temperature |
| Wiring between the inlet thermistor and control board is intact | Inspect the control board for signs of damage or corrosion | No visible damage or corrosion |
Applicable Rinnai Models
- CU(gas)
- HE(dual_fuel)
- HE++ Series(gas)
- I-SERIES(gas)
- NCB(gas)
- NPE(gas)
- RE(gas)
- REP(gas)
- RL(gas)
- RU(gas)
- RUCS(gas)
- RX(gas)
- RXP(gas)
- Rinnai NCB Series(gas)
- Rinnai NPE Series(gas)
- Rinnai R Series(gas)
- Rinnai RL Series(natural_gas)
- Rinnai RU Series(natural_gas)
- Rinnai RUR Series(gas)
- Rinnai VSP(gas)
- V(gas)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Inlet Thermistor | RIN-12010100 | — | Find Part |
| Control Board | RIN-12020000 | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Rinnai model families as 31.
| Code | Description | Severity |
|---|---|---|
| 02 | Stagnant Water Detection | Warning |
| 10 | Air Supply/Exhaust Blockage | Warning |
| 11 | No Ignition | Warning |
| 12 | Flame Failure | Warning |
| 14 | Thermal Fuse | Warning |
| 16 | Over-Temperature | Warning |
| 19 | Electrical Grounding | Warning |
| 25 | Condensate Overflow | Warning |
| 29 | Condensate Neutralizer | Warning |
| 32 | Outlet Thermistor Failure | Warning |