5 nn Low

Keston 5 nn Fault Code

Also written as 5NN — same fault.

The Keston 5 nn fault code means: your boiler is currently recalibrating its internal settings and should return to normal operation automatically once the process is complete.. It appears on 1 Keston model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Temporary mode whilst module adjusts its internal settings

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Keston models that show the 5 nn fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Keston 5 nn fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026