Boiler Fault Code A63 / A64 / F66

Also written as A63A64F66 — same fault.

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

Fault code A63 / A64 / F66 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's internal computer has developed a technical fault and can no longer communicate with the rest of the system, causing it to shut down for safety. (As reported on Ferroli iBoiler 28)