Navien NCB300-28K, Service Manual

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Troubleshooting 75
Check method
Fault Possible causes Testing method
Faulty sensor
Temperature sensor
connection fault
Check if the temperature sensor is open and if the connector is connected
properly.
Temperature sensor fault
Test the resistance of the temperature sensor. The sensor is faulty if the
resistance is 30kΩ or higher.
●
Replace the temperature sensor if the resistance value is abnormal.
●
Check the temperature displayed on the front panel.
Possible Issues
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.