Design validation and verification is the process of ensuring correctness of a design described at different levels of abstraction during the design process. Design validation is the main bottleneck in improving design turnaround time. Currently, simulation is the primary methodology for validation of the first description of a design. In this paper we integrate directed search methods and observability-based code coverage metric (OCCOM) computation into an algorithm for generating test vectors under OCCOM for sequential HDL models. A prototype system for design validation under OCCOM has been built. The system uses repeated coverage computation to minimize the number of vectors generated. Experimental results using the test vector generation system are presented.