Navien NCB300-28K Service Manual page 71

Testing method
Fault
Heat exchanger overheat
Defective part
Other faults
5.2.7 Error 031
70 Troubleshooting
Possible causes
1. High exhaust gas temperatures can cause damage to or obstruct the heat
Damaged or obstructed
heat exchanger 2. Flush the heat exchanger to remove scale deposits.
3. Replace the heat exchanger if it is damaged or cannot be unclogged.
Faulty terminals on the exhaust gas overheat controller (110°C max)
Defective overheat
●
controller
●
●
Test if the hot water temperature sensor is open (error result: MΩ open)
PCB fault If the error condition continues after checking these items, replace the PCB.
Is the burner overheat
prevention disconnected?
No
Check the PCB.
Testing method
exchanger.
Check the overheat controller's wiring connections.
If the resistance is incorrect, replace the temperature sensor.
Check the output temperature displayed on the PCB.
Yes
Replace the burner overheat
prevention.