A fault injection circuit injects a test signal into a data bus with a normal high logic level and a normal low logic level. The test signal has a greater logic level greater than the normal high logic level of the data bus or a lower logic level lower than the normal low logic level of the data bus. An analog-to-digital converter is coupled to a voltage level detector for sensing an aggregate level of an aggregate signal on the data bus. The aggregate signal is composed of the termination circuit signal and the test signal. A diagnostic tool determines whether a faulty connection between the data bus and a network device exists, where the sensed aggregate level exceeds at least one of the normal high logic level and the normal low logic level.