A line of hand-held diagnostic equipment has been developed which provides the automotive service mechanic with the necessary diagnostic information that he needs to allow him to diagnose today's complex electronic engine control computers. This equipment utilizes the on-board diagnostic capability available in the electronic engine control computer as well as the tester's own microprocessor based intelligence. This equipment offers a high degree of sophistication in a very easy to use package with extremely favorable economic incentives. In addition, this equipment offers the capability of being updated easily so that new vehicle model offerings may be readily accomodated. This allows the service facility to keep pace with ever-changing automotive diagnostic requirements.