Navien NCB300-28K, Service Manual

Page: 75 / 108

74 Troubleshooting
5.2.11 Error 205
Error conditions and checklist
Error Description
E205
Heat exchanger output temperature
sensor open
If an error (open: -10°C or lower) in the heat exchanger input temperature sensor is
detected, the system displays 205E on the front panel. If this occurs, the boiler shuts down.
Checklist
1. Check if the heat exchanger output temperature sensor connector is wet due to any
reason, including leakage.
2. Replace the defective heat exchanger output temperature sensor.
3. Check the circulation pump’s operating status and the flow rate in the space heating
lines.
4. Test the voltage at the PCB to verify the correct voltage at the pump. If there is no
voltage, bleed the air from the system before resetting the unit. If the error condition
persists, replace the pump.
Is the inlet thermistor
normal?
Check connection
#5 and #6 at CONE1
Replace outlet thermistor
No
Yes
Before measuring the thermistor’s
resistance, disconnect it from the
PCB.