The Engine Control Unit (ECU) does exactly what its name suggests; it controls the engine. By reading input signals from various sensors, it interprets the signal, then makes adjustments by controlling various output devices, such as the fuel injectors or an EGR solenoid. In this manner, the ECU can monitor and control the air/fuel ratio, ignition timing, idle speed, engine temperature, etc. By doing so, the ECU is able to run the engine at an optimal level at all times, giving fuel economy, power and stringent emission control.