4 nn High

Keston 4 nn Fault Code

Also written as 4NN — same fault.

The Keston 4 nn fault code means: your boiler has detected an issue with the heating zone for a specific part of your home, and the burner is not operating correctly for that section.. It appears on 3 Keston models in our database.

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: Module burning for zone 2
Typical repair cost £150-350 (including labour)

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 4 nn fault code

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

Source

Keston 4 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