Get a garage to come out and fix it. They will put it on a code reader and tell you the problem. simples! By the way, how do you expect help from this site when you can't even to bothered to give details like,make, model, petrol, diesel, year. we are not Mystic Megs
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.