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 units that control the fuel injection system, ignition timing, idle-speed control, air conditioning, EGR systems and 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 often located in the cabin (for example under a seat or behind the dashboard) and may use a blink/flash fault-code system to indicate problems. An ECU can fail with little or no warning; common causes include voltage surges (for instance from incorrect jump-starting or a failing battery or alternator), water ingress, and connector corrosion or vibration damage.