Navien NCB-180 Service Manual page 96

Check method
Fault Possible Causes
Damaged or
Heat exchanger
clogged heat
overheated
exchanger
Defective
overheat
controller
Defective part
Other trouble Defective PCB
1. The error occurs due to high exhaust gas temperature caused by a damaged or
clogged heat exchanger.
2. Flush the heat exchanger to remove scale deposits.
3. Replace the heat exchanger if it is damaged or cannot be unclogged.
Defective contact point of the exhaust gas overheat controller 230°F (110°C) Max
Check connection of the overheat controller.

If the resistance is abnormal, replace the temperature sensor. (refer to page 62)

Check the output temperature displayed on the PCB.

Check if the hot water temperature sensor is open (Error type : MΩ Open)
If the issue continues despite checking the items above, replace the PCB.
Check method