Boiler Fault Code Err.4

Also written as ERR4 — same fault.

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

Fault code Err.4 appears across 1 boiler brand 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 displaying a historical record of an older issue, which means the current problem is likely linked to a fault that happened several times in the past. (As reported on Atag Q 25S)