Boiler Fault Code AL 18

Also written as AL18 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code AL 18 appears across 1 boiler brand and 15 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 waste water cannot drain away properly, usually because the discharge pipe is blocked or the internal trap is full. (As reported on Sime Brava Slim HE System)