Navien NCB-180 Service Manual page 94

Check method
Fault Possible Causes
Defective
overheat
controller
Defective safety
device
Defective hot
Temperature water output
sensor error temperature
Capacity setting
Other potential
Primary heat
issues
exchanger
overheated
Defective PCB
Check if the contact point of the overheat controller is defective.
Use a multi-meter to see if the resistance is normal (0.3Ω) or abnormal (∞).

Check if the overheat controller wire is disconnected ( Normal resistance : 0.3Ω )
1. If the hot water temperature is sensed lower than it actually is due to a defective
sensor, check if the deviation of temperature is large due to a defective temperature
sensor.
Check the output temperature displayed on the front panel.

sensor
2. Measure the temperature sensor resistance, and determine if the sensor is defective.
1. If the Max switch #1 of 1-1 is on,
Set the switch to the normal operation position.
2. PCB DIP S/W capacity setting error can suddenly increase the hot water temperature.
The surface temperature rises due to heavy scale deposits in the primary heat exchanger.
Flush the primary heat exchanger.

If the issue continues despite checking the items above, the PCB is defective.
Check method
Overheat controller