Abstract: An embedded real time diagnostic system that is applicable to fault detection and isolation in host systems having repetitive subsystems. A query directed acyclic graph (Q-DAG) for one of each type of repetitive subsystem is compiled off-line from a causal network model and is stored in host system computer memory. When a fault in the host system is detected by the computer, a replicating program replicates the necessary subsystem Q-DAGs to construct an overall host system Q-DAG. A data table which contains information on how the various Q-DAGs are linked based on the hardware architecture of the host system, is also accessed by the computer. The data table contains a matrix which dictates which Q-DAGs are linked, and Q-DAG level information on the Q-DAG nodes and which nodes are linked to nodes in other Q-DAGs. The new diagnostic system uses less memory and processor throughput while still providing full system diagnostics.