The presented thesis work is centered on the topic of functional test of microprocessors. Functional test, in particular using Software Test Library (STL), is becoming a standard solution for the online test of safety-critical systems, for instance in the automotive domain. Commercial tools provide scan test patterns, however frequently they are not able to reach the given target fault coverage. Nevertheless, the development of high-quality test programs is considerably more challenging than using commercial tools, therefore new solutions are required to improve the fault coverage guaranteed by a STL. The proposed work focuses on the analysis of the propagation of the faults through the paths of the device under test and tries to propose valid solutions to improve the fault coverage of the tests.