Abstract—Past efforts in the development of diesel engines were focused on engine performance and functions. Today, implementation of environmental protection policies by the governments has set new directions for engine development. The fuel efficiency of the CI engine is improved to minimize exhaust gas, and post-processing in exhaust gas systems performance enhanced to reduce the emission of toxic contaminants. A common rail direct injection (CRDI) diesel system injects high-pressure fuel directly into the combustion chamber via Common-rail injector. Through this process, complete combustion is achieved in the combustion chamber. The common-rail injector is a critical in terms of combustion emission. This study examined the characteristics of CRDI injector parts and carried out comparative tests on injectors under normal and abnormal conditions. The tests include a compressive pressure test, idle speed comparison test, and injector correction test. Uniform output between cylinders was observed for the normal injector, whereas cylinders with defective injectors showed (-) non-uniformity. Injector correction involved increasing the amount of injected fuel to compensate for the insufficient force between cylinders. Based on the injector current wave, this method of injector correction was found to increase the energizing time by about 0.2 ms in the idle section. The injector fuel return flow test demonstrated an increase in return flow from defective injectors, which can be traced to internal oil fuel leaks or the higher amount of injected fuel.