The ECU is responsible for controlling many vital engine functions. When it fails, it can wreak havoc and cause a multitude of problems. When resetting the computer with a code scanner or by disconnecting the battery fails to resolve those issues, a replacement ECU is in order. On a Ford Focus, the unit is in the interior of the car, making for an easy fix that can be done by most people.
Open the glove compartment and remove any items contained inside. Place one hand on each side of the box, toward the very top, and press in with firm pressure until the box pops out, making the ECU visible.
Remove the ECU bracket using a socket wrench. Reach behind the unit, depress the plastic tab on the wiring harness and pull straight back to unplug it. Remove the ECU.
Install the replacement ECU by reversing the steps used to remove the old one. Ensure the bracket is tight and the wiring harness is pressed completely into the socket. Close the glove box, pressing in on the sides again to snap it back into place.
Letting the car idle for 10 minutes before driving will allow the new ECU to calibrate itself, leading to smoother engine operation.
Make sure the ignition is turned off before beginning.