1. A diagnostic method in which diagnostic simulation results of current tests are stored in an incremental fault dictionary, and information in the incremental fault dictionary is used to avoid expensive redundant fault simulations, comprising: for faults associated with a failing device, perform fault simulations of failing patterns, while not repeating fault simulations of faults that have already been simulated for any of the failing patterns and whose corresponding results are stored in the incremental fault dictionary, and complete the current diagnosis for the failing device, and add the diagnostic simulation results for the current diagnosis to the incremental fault dictionary if not already stored in the dictionary.