Most likely you have a faulty fan element- this is a round element that goes around the fan at the back. To check- Disconnect from power(easiest way is to switch off oven at fuse board). Remove rear panel inside oven (held on by screws), and visually check element- often you will see a blow mark on the element. Element is easily changed, although sometimes requires oven to be removed to access element wires from the back. Proceed if you are a confident DIYer, otherwise call a local service agent. Elements are readily available for $75-$125 depending on the retailer.