In automotive electronics, an Electronic Control Unit (ECU) is an embedded system that controls one or more of the electrical systems or subsystems in a vehicle. Some modern cars have dozens of ECUs, including: controls the fuel injection system, ignition timing, idle speed control system, air conditioning, EGR systems, power to the fuel pump (through the control relay).
Based on information from the input sensors (engine coolant temperature, barometric pressure, air flow, etc.), the ECU determines optimum settings for the output actuators (injection, idle speed, ignition timing, etc.).
The ECU is typically located under one of the seats in the car and uses a light code system to indicate faults in the system. In Most cars the ECU will fail completely with no advance warning. That being said, the number one cause of an electronic control unit failing is that the car or truck was jump-started using cheap jumper cables causing the electrical system to surge after the vehicle is started.