The paper discusses the use of digital computers in developing dynamic tests for testing a control system during its actual operation, or for a test rig which simulates the system's working conditions. Usually three levels are required in any successful test scheme: a check-out test to classify the system as faulty or non-faulty, a diagnostic test to locate the parameter or component responsible for the fault and finally an estimation test to establish a quantitative measure for the faulty component before it is repaired or replaced. The scheme is applied in a case study of a throttle control servosystem, where a digital model of the actual system used as a vehicle for the study.