In an automotive distributed embedded system direct access to individual nodes for testing is often impossible during field operation. Remote test access over the communication network -albeit suboptimal from a testability point of view -is sometimes the only viable solution from an economic point of view. At the example of the FlexRay communication protocol we investigate which diagnostic conclusions with respect to the communication services can be drawn from remotely monitoring the normal

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... traffic. In particular we discuss how the the clock synchronization service can be exploited for online diagnosis. We further propose a novel approach for transparent online testing that utilizes the clock synchronization services to loop back information on a node's reception status to the tester.