Onboard diagnostics (OBD) have been an integral part of vehicle design for decades, as OBD-II became mandatory in 1996 for all cars manufactured in the United States. Almost every aspect of the diagnostics system evolved from emissions requirements placed on the automotive industry over the years. Manufacturers responded with increasing complexity in the electronic controls and reporting of errors. Fortunately, some commonization followed in the industry, making the job of a repair technician or do-it-yourself enthusiast slightly easier when working on these challenging systems. Passing emissions has also become critically important to manufacturers of high-performance parts and installation shops in recent years. Both the US Environmental Protection Agency (EPA) and California Air Resources Board (CARB) have assessed massive fines for violations of the Clean Air Act based largely on information available from the vehicle's OBD system. Many states now require that the vehicle be able to pass an OBD check to renew the registration, making proper operation even more important to consumers. OBD-I & OBD-II: A Complete Guide to Diagnosis, Repair, & Emissions Compliance is an insider's guide to modern original equipment manufacturer (OEM) diagnostics systems. As a former diagnostics and performance calibration engineer, Greg Banish guides you through critical ECU functions that ultimately lead to why the "check engine" light on the dash is lit. Strategies are given for how to begin looking for a solution when problems are detected. Listing every imaginable repair is impossible, but this book provides a solid foundation from which to work by explaining the electronic control unit's (ECU) logic in plain English. Practical examples from the author's experience highlight important lessons that will eliminate hours of frustration and confusion. In addition to repair tips, sound advice is provided for those looking to modify their vehicles while maintaining the full functionality of the emissions and diagnostics systems. Modern diagnostics systems can be a blessing or a curse, depending on your level of understanding. This book shows that by using commonly available electronic tools, the consumer has the power to access a wealth of information about his or her vehicle. It also helps those who are looking to improve performance do so without compromise to the environment or enforcement agencies. Whether you want to repair a vehicle yourself or you simply want to possess an understanding of how these systems work while dealing with service professionals, this is the guide you need.