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All Toyota vehicles with electronic fuel injection utilize the Toyota EFI or TCCS engine management system. This system incorporates many features to enhance performance, emissions, and drivability. One feature is the Self Diagnostic System. The self diagnostic system monitors engine operations and when a particular sensor or input varies out of specified operating parmeters, the "Check Engine" will be illuminated. The check engine lamp alerts the driver or technician that a problem exists in the EFI/TCCS ECU or Engine Control Unit or "computer." When the check engine light is illuminated, it is indicating that a problem in the engine management system has been detected. At this time, a "code" may be stored in the ECU which pinpoints a possible component in the EFI/TCCS system. By checking the trouble codes, a possible problem could be narrowed down to a number of causes. While not an exact diagnostic tool, the check engine light and self-diagnostic system can get the individual working on his/her truck close to a possible fault, and, hopefully, to a successful repair