In a computerized automotive vehicle diagnostic system, a test probe cable is attached to individual components and consists of a multiple conductor cable which is attached ultimately to the component and to a computer in a technician terminal. Where the probe cable enters the technician terminal the conductors contained within it are fed into a programmable crosspoint switch which is controlled by a microcontroller. Test functions are provided by three programmable resources: (1) A program controlled multimeter, (2) A program controlled DC voltage source and ground sink; and (3) the microcontroller itself which receives and interprets commands from the computer and controls the various probe elements based upon the test commands received.