Boiler Fault Code 8 04

Also written as 804 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026

Fault code 8 04 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 is signalling that an additional electronic control module, required for managing specific heating zones or complex setups, has not been detected or is not communicating correctly. (As reported on Ariston Genus ONE Net)