ECU is normally an OEM item. comes together with the car from factory.
EFI is actually an intelligent system, designed to give optimum performance with good fuel consumption in return.
ECU is needed to calculate how much fuel must be sprayed into the combustion chamber.
the more the loading, the more fuel needs to be sprayed. but how? and when?
sensors do the job. for example water temperature sensor, cold start sensor, lambda sensor, throttle position sensor, camshaft position sensor (to determine piston TDC).
air flow sensor among others.
imagine like this.....he he he
in one classroom the teacher is the ECU. students are the sensors. when one student dont submit his homework, the teacher will take action rite? same laa..with engine.
when engine is cold, sensor will tell the ECU and ECU will increase the engine speed just like auto choke. ECU also function to reduce engine knocking during acceleration.