BoilerFaultCodes ALL 18 High
Boiler Fault Code ALL 18
Also written as ALL18 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code ALL 18 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 has detected that water produced during the heating process is not draining away properly and is backing up inside the system. (As reported on Sime Murelle Advanced HE Combi)
Pick your boiler brand for the ALL 18 fix
Source
Information for the ALL 18 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