Boiler Fault Code 1 09

Also written as 109 — same fault.

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

Fault code 1 09 appears across 1 boiler brand and 8 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 pressure inside your heating system is too high, which can cause internal leaks or force the safety valve to open. (As reported on Ariston Clas Net One)