My Baxi solo 3 PFL boiler will not fire up when on central heating only ?

My Baxi solo 3 will not fire up when set to central heating only. Will fire up when set to hot water only and when set to hot water and central heating on timer. Have had new cistern because of constant over flow leak. New pump because of boiler over heating and new 3 postion port valve because boiler was over heating this has now solved the over heating problem but now left with the fire up problem. Could it be that I need a new programer?