Navien NCB-180 Service Manual page 114

Check method
Fault Possible Causes
temperature sensor
Defective sensor
Temperature sensor
Defective PCB
Possible Issues
Defective
Check if the temperature sensor is open and if the connector is connected properly.
connector
Check the resistance of the temperature sensor. (Defective if it is 30kΩ or higher)
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If the issues continue despite checking the items above, replace the PCB.
<DHW Flow sensor input temperature sensor / connector>
Check if the hot water temperature sensor is open (Error type : MΩ Open)
Check method
Replace the temperature sensor if the resistance value is abnormal.
(refer to page 62)
Check the temperature displayed on the front panel.
(refer to page 39