hotpoint minstral ff fan not working!!!?

Having similar problems as others with the hotpoint minstral frost free fridge freezer. freezer wqorking great, fridge warm. bought new yellow thermistor. put it in and fan at back of inside freezer now only spins for seconds then nothing...what do I do know?
bought a thermistor about 2 years ago and its lasted this long. but bought a new one again and thought it would solve the problem..maybe something else now? can't afford a new fridge.