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No trouble found (NTF) due to functional failures is a common scenario today in board-level testing at system companies. A component on a board fails during the board-level functional test, but it passes the automatic test equipment (ATE) test when it is returned to the supplier for warranty replacement or service repair. To find the root cause of NTF, we propose an innovative functional test approach and discrete Fourier transform (DFT) methods for the detection of board-level functional failures. These DFT and test methods allow us to reproduce and detect functional failures in a controlled deterministic environment, and provide ATE tests to the supplier for early screening of defective parts. Experiments on an industry design show that the proposed functional scan test with appropriate functional constraints can adequately mimic the functional state space, as measured by appropriate coverage metrics. Experiments also show that most functional failures due to dominant bridging, crosstalk, and delay faults due to power supply noise can be reproduced and detected by functional scan test. We also describe two approaches to enhance the mimicking of the functional state space. The first approach allows us to select a given number of initial states in linear time and functional scan tests resulting from these selected states are used to mimic the functional state space. The second approach is based on controlled state injection.