How much does it cost to fix Keston error code 9 nn?

Typical repair cost for the Keston 9 nn fault is £150-350 including labour.

9 nn High

Keston 9 nn Fault Code

Q: What does the Keston 9 nn fault code mean?

Your boiler has detected an internal error it does not recognise and has shut itself down as a safety precaution.

Also written as 9NN — same fault.

The Keston 9 nn fault code means: your boiler has detected an internal error it does not recognise and has shut itself down as a safety precaution.. It appears on 1 Keston model in our database.

Technical description: Module off due to unexpected problem

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 9 nn fault code

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

C90 / C110 Repair: £150-350

Source

Keston 9 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