The on-board computer in your 2003 Ford Focus is programmed to control the transmission, fuel system, and ignition system in your car. It also monitors engine, transmission, and emission control systems to provide the best performance, fuel mileage and emissions. This device is fairly robust but can be easily damaged by improper diagnostic procedures or stereo installation. Replacement of the computer is straight forward and can be done in about 30 minutes by most home mechanics.
Disconnect the negative battery cable from the battery using a 10-mm wrench to loosen the bolt that clamps the terminal to the battery. In the 2003 Ford Focus the battery is located on the right side of the engine compartment between the air filter housing and the radiator core support.
Open the glove box and remove it's contents. Release the box from the dash by pressing inward on the upper corners of the box until the latches release and it opens completely. The computer in the Focus is located behind the glove box and attached to the dash frame.
Unbolt the computer bracket from the dash frame and position the computer so you can access the electrical connector. Unplug the computer's electrical connector and slide the connector out of the end of the computer. Remove the loose computer from the dash and unsnap the computer from the bracket.
Install the new computer into the computer bracket. Position the computer inside the dash and plug the electrical connector into the new computer until it snaps into place. Bolt the computer bracket to the dash frame.
Close the glove box door. Reconnect the negative battery cable to the battery and tighten the 10-mm terminal bolt securely. Start the engine and allow it to idle for 15 minutes before test driving the Focus to verify that the malfunction is repaired.
