Code 31
Rinnai Inlet Thermistor Fault

Unit will not operate and displays Code 31

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Open or shorted inlet thermistor, Loose thermistor wiring, Corroded connector at control board, Faulty control board input

How to Fix / Troubleshooting

Safety first: Turn OFF power and gas before removing covers. Thermistors are low-voltage components but are connected to the control board.

Steps to check:

  • Locate inlet thermistor: Remove the front cover and find the thermistor attached to the cold water inlet pipe or near the heat exchanger inlet.
  • Inspect wiring: Ensure the thermistor harness is firmly connected at both the sensor and the control board. Look for damaged or pinched wires.
  • Check for corrosion: If connectors are corroded, they may need cleaning or replacement by a technician.
  • Reset and test: After reseating connectors, restore power and gas and reset the unit.

When to call a professional: If Code 31 remains, the inlet thermistor likely needs replacement and the control board may require testing.

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Repair Difficulty

Medium 3/5

Required Part

Inlet Thermistor
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