High
E439 on Navien NCB CE

Your boiler is struggling to detect the movement of water, which means it cannot safely start heating your home or providing hot water.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Flow Sensor: Unable to detect or measure water flow rate; Damage to and/or leakage from the water flow sensor

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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