Assuming you have taken your car to the supplying dealer and they have diagnosed the fault, we would suggest giving them at least one opportunity to repair the fault. However, we would suggest keeping a record of all communication with the dealer in the event that the fault cannot be repaired and you wish to reject the vehicle. You can read more about how to do this here: www.honestjohn.co.uk/how-to-reject-a-car-your-cons.../