DIAGNOSTIC SYSTEM PARTICULARLY FOR AN ENGINE MANAGEMENT SYSTEM

A diagnostic system (10) particularly though not exclusively for use in an engine management system is provided for generating a diagnostic trouble code (DTC) to indicate the operational status of a component or sub-system which is being evaluated by the diagnostic system. The diagnostic system includes a diagnostic function module (DF Module) (20, 20', 20") for each DTC or a group of related DTCs associated with a component or sub-system. The DF module including device (22) for executing an evaluation routine to evaluate the operational status of a component or sub-system to which the DTC of the specific DF module relates, and a diagnostic function scheduler (DF scheduler)(30) for determining which DF module may be allowed to execute an evaluation routine ant a particular time. Each DF module (20, 20', 20") includes device (22) for producing a ranking value dependent on the operating status of the component or sub-system being evaluated, a ranking value being generated each time an evaluation routine is performed; device (23) for processing and storing statistical results of the ranking values obtained over a number of evaluation routines; device (23) for evaluating the statistical results to produce evaluated data in the form of either an evaluated no-fault signal or an evaluated fault signal, and device (24) for transmitting the evaluated signals to the DF scheduler (30).