Navien NCB300-28K, Service Manual

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System Details 39
4.7.3 Thermistor
Item Description
Function Measure hot and cold water temperature at the boiler’s space heating outlet and inlet connections.
Fault Inaccurate water temperatures from inside the boiler.
Symptoms
1. If a thermistor fails, an error code is displayed before the boiler operates.
2. If resistance values are outside the allowable range, there will be hot water temperature fluctuations.
Error codes E047, E205, E218, E407, E421
Diagnostics
1. Visual inspection: Check wire connections are secure and inspect for wire damage.
2. Test the resistance of the sensor. Before testing, shut down the boiler and allow it to cool.
Testing/inspection
information
Resistance range: Refer to the table below
Wire colour: BLUE-BLUE
Inlet, Outlet
Exhaust limit temperature sensor
Wire colour: BLUE-BLUE
Test for an open hot water temperature sensor (Test result: MΩ open)