Boiler Fault Code 8 (Diagnostic)

Also written as 8DIAGNOSTIC — same fault.

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

Fault code 8 (Diagnostic) 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 has detected a faulty sensor that monitors the internal water levels, causing it to shut down to prevent potential overheating. (As reported on Potterton Envoy 30)