1. A method for verifying, validating and improving a fault model, said method comprising: providing an initial fault model that identifies a relationship between symptoms in a vehicle and failure modes in the vehicle; providing field failure data that includes vehicle symptoms and vehicle failure modes of many vehicles that are in the field being operated; performing a what-if analysis using the field failure data that includes using subject matter expert (SME) knowledge to determine the most significant failure modes and the most significant symptoms; learning simulation parameters from the field failure data; simulating faults using the learned simulation parameters; revising the initial fault model using the results of the what-if analysis and the simulations; using a diagnostic reasoner to analyze the revised fault model to generate estimated faults; comparing the estimated faults to the simulated faults to determine true detection and false alarm rates; and performing a benefit analysis by relating the true detection and false alarm rate to costs.