Weak testability[1] is a testability measure for register-transfer level (RTL) data paths. If a data path satisfies weak testability, for each hardware element of the data path, there exist paths from some primary inputs to the element to justify some values on its output and paths from the hardware element to some primary outputs to propagate some values on its output. For a weakly testable data path, a sequential ATPG tool can generate a test sequence with high fault efficiency in short test generation time[1]. In this paper, we introduce a notion called test knowledge, use of which by commercial ATPG tools can further decrease the test generation time and increase the fault efficiency in weakly testable data paths. This test knowledge is information which is relevant to structure of weakly testable data paths, and also easy to find. Use of this test knowledge to facilitate the test generation and increase the testability of data path is established in experimental results.