How much does it cost to fix boiler error code 4 nn?

Typical repair cost for the boiler 4 nn fault is £150-350 including labour.

4 nn High

Boiler Fault Code 4 nn

Also written as 4NN — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code 4 nn appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning 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. (As reported on Keston C90 / C110)

Pick your boiler brand for the 4 nn fix

Keston 4 nn 1 model

Source

Information for the 4 nn fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

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