A method and a system determines failure of a urea quality sensor. The method may include: determining whether a urea temperature is higher than a predetermined temperature; determining, if the urea temperature is higher than the predetermined temperature, whether a vehicle speed is faster than a predetermined vehicle speed; determining, if the vehicle speed is faster than the predetermined vehicle speed, whether a difference between a current urea quality and a previous urea quality is higher than a predetermined urea quality; and storing a failure code if the difference between the current urea quality and the previous urea quality is higher than the predetermined urea quality.