Teka HTD888 double oven - lower oven started covecting cold air ?

I have a Teka HTD888 double oven which has devel;oped a strange action for which I cannot find out what needs replacing/fixing. The lower oven has just started convecting cold air yet the heater element still works but only when set to grill. In theory all the parts work just not necessarily together. Can you advise answer or where I can find necessary info?