In order to save money, many car owners directly shield SCR system, so as to achieve the goal of adding less or no urea? The result is tragedy. Carbon deposit is serious. Repairing a car is a big expense, etc. Since the emission standard of diesel vehicles has been raised from country III to country IV, the selective catalytic reduction (SCR) technology is generally used to treat the nitrogen oxides in the exhaust gas by using urea solution for vehicle, so that the nitrogen oxides can be converted into harmless nitrogen and water. During the use of Guo IV diesel vehicle, the amount of urea used is 3%~5% of the fuel consumption. Each heavy truck of Guo IV standard consumes about 2 tons of urea per year.