A diagnostic test interface circuit and method for using the same are disclosed. The circuit isolates communication links at a junction box having one pin for each of the communication links. The circuit includes a bus, a plurality of switches, and a plurality of connection leads. The switches each have a first terminal connected to the bus. Each connection lead includes a first end having a connector that connects to one of the pins and a second end that connects to a corresponding one of the switches such that the switch connects the connection lead to the bus when the switch is closed. The circuit can be used to diagnose a fault in a vehicle having a plurality of modules that communicate over a star-configured network.