ON every 0.5-secs (YELLOW LED) Low

Boiler Fault Code ON every 0.5-secs (YELLOW LED)

Also written as ONEVERY05SECSYELLOWLED — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code ON every 0.5-secs (YELLOW LED) 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 is currently running in a special diagnostic test mode used by heating engineers, rather than a normal heating mode. (As reported on Vokera Compact HE)

Pick your boiler brand for the ON every 0.5-secs (YELLOW LED) fix

Vokera ON every 0.5-secs (YELLOW LED) 1 model

Source

Information for the ON every 0.5-secs (YELLOW LED) fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

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