Your boiler is switched on and ready to work, but it isn't responding when you turn up the heating or hot water, usually because an external controller or a valve is failing to tell the boiler to start.

Boiler ON but not operating during demand. Possible solutions/checks: Live demand at ST2 terminal L - is there a demand?, Manual bypass is open. If CH is working check HW controls, If HW is working check CH controls (i.e. external diverter valve, frost stat, room stat/programmer. Otherwise replace board.