a diagnostic tool for testing fuel injectors and their driver circuits , including associated wiring harnesses , in a fuel injection system of an engine in a motor vehicle . the tool has a driver - connection port for connection to the driver circuits and an injector - connection port for connection to the fuel injectors via jumper wiring harnesses . the tool can perform 1 ) a fuel injector test , by connecting an ohmmeter to the tool to measure injector coil resistance , 2 ) a driver - injector test , by connecting an oscilloscope to the tool to observe traces of electric current waveforms applied to the fuel injector , 3 ) a driver circuit test , by connecting the respective driver circuit to a load that simulates the load imposed by the respective fuel injector , and 4 ) a cylinder contribution test , by selectively disconnecting certain fuel injector actuators from their driver circuits while leaving the remaining fuel injector actuators connected to their driver circuits .