Boiler Fault Code A.(4)

Also written as A4 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026

Fault code A.(4) appears across 2 boiler brands and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is unable to start because the small pilot flame that ignites the main burner cannot be lit. (As reported on Potterton Kingfisher 2 RS60, RS80, RS100)