Ensuring the reliable operation of the emissions control system is a critical factor in complying with increasingly stringent exhaust emissions standards. In spite of significant advances, the performance of available diagnostic and test equipment is still amenable to further improvement, especially as it pertains to the diagnosis of incipient and intermittent faults.This paper presents experimental results pertaining to the diagnosis of complete, partial and intermittent faults in various components of the engine emissions control system. The instrumentation used in the study permitted simultaneous and essentially continuous analysis of the exhaust gases and of engine variables. Tests were conducted using a section of the EPA urban driving cycle (I/M 240), simulated by means of a throttle/dynamometer controller. Various faults were induced in the engine control system (e.g., vacuum leaks, misfire, miscalibrated injectors) in an attempt to approximate realistic fault conditions and their effects on the exhaust emissions during the I/M 240 cycle were quantified.The paper also proposes procedures capable of isolating the defective component(s) using model-based fault detection strategies in conjunction with a set of logical rules, representing a compilation of available diagnostic and design experience. The overall aim of this study is to provide a useful framework for the design of diagnostic instruments that will help in the improvement of the quality of high volume emissions inspection lanes and to provide accurate and reliable diagnostic information.