Boiler Fault Code Error 18

Also written as ERROR18 — same fault.

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

Fault code Error 18 appears across 1 boiler brand and 5 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 the water inside the system has become dangerously hot and has shut itself down to prevent damage or overheating. (As reported on Atag Q 25CR)