Abstract: An apparatus and method for testing electrical loads such as appliances connected with a DC source in a motor vehicle. The loads receive their signals causing them to operate from an electronic control unit. The latter is connected with a diagnostic computer in a manner so as to comply with the industrial standard DIN/ISO 9141. The diagnostic program is stored in the computer, for instance on a diskette. During a test, a precision resistor or shunt is connected in the current path between the DC source and the load. When the load to be tested has been turned on by the diagnostic computer the precision resistor leads to produces a voltage drop dependent on the current flow in the load. Such voltage drop is measured and compared in the diagnostic computer with a rated value specific for the load. If there is a difference the size of the difference provides information indicating the nature of the fault or malfunction of the load.