Fat burn or Cardio issue?

Hi all ive recently started to go back to the gym to shift a few stone :P ive been going on and off for years (6 months at a time but never asked this question that i always wanted to know) on the machines i can choose workouts as you all may know, ive noticed that fat burn workouts are low intensity and cardio is high this also chages my target heart rate, if say when on a X trainer my THR should be130 (im 23 hears old) if i go over this into the cardio zone or say up to 150 will i no longer burn fat? and how can i stop going over? because if i slow down or make it harder / higer level it still goes up... any help would be great. thanks scott