Boiler Fault Code RED blinking (0.5s)

Also written as REDBLINKING05S — same fault.

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

Fault code RED blinking (0.5s) appears across 1 boiler brand and 5 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 signal that it is already burning fuel even though it has not been told to start, so it has safely shut itself down to prevent an unsafe situation. (As reported on Grant Vortex Eco)