Navien NCB500-2S/37K Service Manual page 80

Check method
Fault
Faulty sensor
Possible Issues
Possible causes
Temperature sensor Check if the temperature sensor is open and if the connector is connected
connection fault properly.
Test the resistance of the temperature sensor. The sensor is faulty if the
resistance is 30kΩ or higher.
Temperature sensor fault
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Circulation pump fault Test the voltage at the circulation pump (AC 230 V).
PCB fault If the error condition continues after checking these items, replace the PCB.
<Heat exchanger output temperature sensor / connector>
Test to confirm if the secondary water temperature sensor is open. Error: MΩ open circuit.
Testing method
Replace the temperature sensor if the resistance value is abnormal.
Check the temperature displayed on the front panel.
Troubleshooting 79