What is causing the problem and can it be fixed?

My oven is working fine and after each use the fan would come on for approx.5 minutes however now the fan stays on. The only way it goes off is if I mess with the timer button and allow the timer to bleep then turn the timer off i.e. press the timer alarm button. The fan then stops but this is happening every ime the oven or grill is used. It is a bit of a nuisance. What is causing this to happen and can it be fixed?