BoilerFaultCodes 1 nn Low
Boiler Fault Code 1 nn
Also written as 1NN — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 1 nn appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning 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. (As reported on Keston C40)
Pick your boiler brand for the 1 nn fix
Source
Information for the 1 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 7 July 2026