Navien NCB-180, Service Manual

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4.9.10 Flow Sensor
Part Check Point
Function
To detect water flow in GPM (Gallons Per Minute) for steady hot water temperatures.
Failure Event
1. Unable to detect or measure water flow rate.
2. Damage to and/or water leakage from the water flow sensor.
Effects
1. Ignition sequence does not start.
2. Stop operating once detecting water leakage.
Error Code
E439
Diagnostic
1. Visual inspection: Connection and/or breakage of wires.
2. Visual Inspection: Check for damage and/or scale formation on sensor.
Color/Number of wires

RED-BLACK: DC 12 V

WHITE-BLACK Start position: DC 0 V
Any open position: DC 5 V

Blue-Brown: Pulse