In trying to test our new Convection oven (Model TSSTTVDGXL) the instructions give us a choice to Bake or Convection Bake. The instructions for Convection Bake read "Press the convection bake button until convection bake lights up on the display". The problem is there is no convection bake button. There is a turbo convection button, but pressing it does not light up the display either. This button is not mentioned in the instructions. Also, it is not clear if preheat is necessary to bake or convection bake. Please Clarify!!