Navien NCB300-28K, Service Manual

Page: 71 / 108

70 Troubleshooting
Testing method
Fault Possible causes Testing method
Heat exchanger overheat
Damaged or obstructed
heat exchanger
1. High exhaust gas temperatures can cause damage to or obstruct the 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 part
Defective overheat
controller
Faulty terminals on the exhaust gas overheat controller (110°C max)
●
Check the overheat controller’s wiring connections.
●
If the resistance is incorrect, replace the temperature sensor.
●
Check the output temperature displayed on the PCB.
Test if the hot water temperature sensor is open (error result: MΩ open)
Other faults PCB fault If the error condition continues after checking these items, replace the PCB.
5.2.7 Error 031
Is the burner overheat
prevention disconnected?
Check the PCB.
Replace the burner overheat
prevention.
Yes
No