BoilerFaultCodes 300 mm Emergency
Boiler Fault Code 300 mm
Also written as 300MM — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 300 mm 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 that it is positioned too close to a window or ventilation point, which poses a safety risk regarding airflow and flue gases. (As reported on Malvern TenTwenty)
Pick your boiler brand for the 300 mm fix
Source
Information for the 300 mm 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