Abstract: A diagnostic test apparatus for simulating pin open circuit, pin short to ground and pin shorted-to-voltage fault conditions at each pin of each connector of an engine controller module (ECM). The apparatus includes a parallel port coupled to a demultiplexer circuit. The outputs of the demultiplexer circuit drive a plurality of relay drivers, which each, in turn, control independently a single relay coupled between one particular pin of an ECM connector and its associated circuit line on the vehicle wiring harness, when the apparatus is coupled in line between the ECM connector and the vehicle wiring harness. The apparatus may be controlled by an external computer such that the three above-mentioned fault conditions may be initiated automatically.