24i junior problem?

boiler hung and fitted all connections good filled and vented system and boiler .no external controls. therefore a link between the lr and ls was made .fired boiler .ok hot water perfect.but no h.deselected hw. ran ch only boiler fired but,no water circulated the tops of the flow got hot but didnt move.cchecked valves ,they were open.boiler ran for 20/ 30 secondsthenstopped.green flame indicator went out.no lockout light ,nor any flashing lights. boiler blue unaltered.fault guide suggested programmer,but that works otherwise it wouldnt start.the control panel,but again the thing does start but only for 20/30 second period. to me it seems that the diverter valvemay be the problem.how with out removing this can i determine if i am correct?