An Engine Control System, or ECU, is a computerized system responsible for managing the engine's performance and fuel efficiency. The ECU monitors and controls the engine's air-fuel ratio, ignition timing, exhaust gas recirculation, and other parameters in order to optimize the engine's operation. It also provides diagnostic information about the engine's performance and can detect potential issues before they become costly repairs. An ECU is essential for modern vehicles to ensure optimal performance and fuel efficiency.