High
E439 on Navien NCB

Your boiler is currently unable to tell if water is moving through the system, so it has safely shut down to prevent damage.

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

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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