1. A method for troubleshooting a fault to determine a root cause of the fault, comprising: creating a Bayesian network model based on information obtained from a Fault Isolation Manual (FIM), where said FIM provides tests to be performed in troubleshooting said fault to determine a root cause of said fault; using heuristics to determine a structure and conditional probabilities for the Bayesian network; imputing a plurality of test costs inherent in the FIM by first generating a plurality of constraints between the cost of each test and fault probabilities that hold for all fault scenarios; using a linear programming algorithm to solve these for said plurality of constraints, and to construct a tuned Bayesian network model; and using the tuned Bayesian network model to iteratively rank likely faults according to their probabilities given accumulating test evidence, and to rank pending tests according to their value.