31
Ruud
Inlet Thermistor Fault
Ruud tankless water heater displays error code 31; unit may not start or may shut down.
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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, Damaged thermistor wiring, Poor connector contact at control board
How to Fix / Troubleshooting
Safety first: Turn OFF power and gas before opening the unit.
- Step 1 – Locate inlet thermistor: Remove the front cover and find the thermistor on the cold water inlet pipe.
- Step 2 – Inspect wiring: Check for cuts, burns, or loose connectors. Reseat the connector at both the thermistor and control board ends.
- Step 3 – Replace thermistor: If the sensor appears damaged or tests open/shorted with a multimeter, remove its retaining clip or screw and install a new OEM Ruud inlet thermistor. Reconnect wiring.
Restore power and test. If error 31 remains, the control board thermistor circuit may be faulty and should be checked by a technician.
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Repair Difficulty
Medium
3/5
Required Part
Inlet Thermistor
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