How much does it cost to fix Keston error code 3 nn?

Typical repair cost for the Keston 3 nn fault is £150-300 including labour.

3 nn High

Keston 3 nn Fault Code

Q: What does the Keston 3 nn fault code mean?

Your boiler has identified an issue with the internal ignition or flame process for your primary heating circuit.

Also written as 3NN — same fault.

The Keston 3 nn fault code means: your boiler has identified an issue with the internal ignition or flame process for your primary heating circuit.. It appears on 1 Keston model in our database.

Technical description: Module burning for zone 1

Typical repair cost £150-300 (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 3 nn fault code

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

C90 / C110 Repair: £150-300

Source

Keston 3 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