An ECU (Electronic Control Unit) is a small device in a car that is responsible for controlling a specific function. It is essentially a computer, a switching system, and power management system in a very small case. The ECU is often referred to as the "brain" of the engine, and it controls many functions on modern cars. The ECUs main job is to keep the engine working smoothly, and it can control the amount of fuel that enters the engines cylinders, which optimizes the fuel-air mix in the cylinder when its compressed, maximizing the cars power and reducing the amount of fuel wasted. The ECU receives inputs from different parts of the vehicle, depending on its function, and it communicates to actuators to perform an action based on the inputs.

The ECU controls many functions on modern cars, and the effects of a faulty ECU can vary quite a bit. For example, the engine warning light may appear on the dashboard, the cars fuel economy and performance could get worse, and the engine may start stalling or refuse to start. While a modern car may be drivable with a faulty or defective ECU, it will be impossible to drive one without an ECU at all.

The ECU is used to operate the engine by using input sensors and output components to control all engine functions. There are different ECUs used for different systems on the vehicle, and some vehicles may incorporate more than one ECU into a single unit called a powertrain control module (PCM) . The ECU is also useful for managing various important systems in a car, such as identifying car keys, adjusting the performance of the engine cooling fan, turning on the car alarm when the car is locked, and more.