Boiler Fault Code F5/F9

Also written as F5F9 — same fault.

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

Fault code F5/F9 appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is failing to start because it cannot detect that the internal fan, which clears exhaust gases, is spinning correctly. (As reported on Viessmann Vitodens 100-W)