A diagnostic method for an exhaust pressure regulator provided in an exhaust system connected to a turbocharged internal combustion engine includes demanding engine brake, measuring a back pressure in the exhaust system by a back pressure sensor provided between the exhaust pressure regulator and a turbo unit when demanding engine brake, measuring the boost pressure by a boost pressure sensor provided in the inlet manifold when demanding engine brake, comparing the measured back pressure and the measured boost pressure with stored back pressure and boost pressure values in order to determine whether there is a defective pressure regulator or a defective backpressure sensor. A diagnostic apparatus for an exhaust pressure regulator provided in an exhaust system connected to a turbocharged internal combustion engine is also provided.