Navien NCB-700-2S+/42K, Service Manual, Page: 53 / 121

52 System Details
4.7.10 Flow Sensor (to be revised)
Item Description
Function To detect water flow in LPM (litres per minute) to provide a steady hot water temperature.
Fault
1. Unable to detect or measure water flow rate.
2. Damage to and/or leakage from the water flow sensor.
Symptoms
1. Ignition does not start.
2. Boiler operation stops when water leakage is detected.
Error codes E439
Diagnostics
1. Visual inspection: Check wire connections are secure and inspect for wire damage.
2. Visual Inspection: Check for sensor damage and for scale forming on the sensor.