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We introduce a scheme for passive node-level fault detection in a distributed system. With each system node associate a low-cost, low-complexity observer which monitors the pattern of incoming and outgoing messages and compares it against an abstracted model of the node's behavior. We develop a fault detection procedure, which is probabilistic because of nondeterminism in the simplified node model. Abstraction reduces model complexity, but renders some errors undetectable by the observer. In the paper we characterize these undetectable errors. Succeeding studies show how to select model abstractions to lower the number of undetectable errors.