A self-diagnosing method for diagnosing a selective catalytic reduction (SCR) system of a vehicle that includes an internal combustion engine. An SCR system is arranged downstream of the engine, and the SCR system includes at least one exhaust gas sensor that is sensitive to nitrogen oxides (NOx) gas. The method includes a first diagnosing sequence (S1) of ensuring that the vehicle is in a stationary state, controlling the engine to operate in a high NOx output engine operating state and in a low NOx output engine operating state, registering an output signal of the at least one exhaust gas sensor when the engine operates in each of the high NOx output engine operating state and low NOx output engine operating state, and diagnosing NOx measurement performance of the at least one exhaust gas sensor on the basis of the registered sensor output.

Disclosed is an exhaust gas purification apparatus for purifying exhaust gas emitted from an engine and discharges the exhaust gas to an outside, the apparatus including: a body formed with an opening area in which one area is opened; an intake part disposed on one side of the body through which the exhaust gas is introduced; a filter disposed inside the body and capable of entry and exit through the opening area; a body cover capable of covering the opening area; a selective reduction catalyst disposed with the body to reduce nitrogen oxides contained in exhaust gas passing through the filter; and a discharge part through which exhaust gas having passed through the selective reduction catalyst is discharged.