1 nn Low

Keston 1 nn Fault Code

Also written as 1NN — same fault.

The Keston 1 nn fault code means: your boiler is currently performing a standard pre-ignition safety check by running its internal fan to clear the system, which is a normal part of the start-up sequence.. It appears on 3 Keston models 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: Boiler is running fan prior to attempting ignition

DIY fix possible

Wait 5 minutes to see if the boiler completes the start-up sequence automaticallyCheck if the display changes to a normal operating temperature after the fan stopsIf the code persists for more than 15 minutes, perform a soft reset of the boiler

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Keston models that show the 1 nn fault code

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

Source

Keston 1 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 7 July 2026