The PCM, power train control module, within your 2003 Ford Focus manages the engine control system. Replacement of the PCM is not very common, but when your Focus is diagnosed with a failing PCM it's important to replace it immediately to ensure the Focus continues to operate. The PCM is a silver box with large electrical connectors coming from it. You can replace the PCM without the help of a mechanic, but you must first purchase the correct PCM for your car.
Place the Focus in "Park" and remove the key from the ignition. Never attempt to work on an electronic component such as the PCM with the key in the ignition.
Open the passenger glove box; squeeze the left and right side of the glove box until the glove box door disengages. You will now see the PCM. Remove the glove box door completely by removing the three screws along the bottom. Remove any trim or carpet needed to access the PCM by moving it out of the way.
Remove the wiring harness bolt using a 10mm socket; the bolt is found on the top of the PCM. Disconnect each electrical connection by simply pulling them out of the PCM. Detach the PCM securing bracket. Unscrew any screws that are securing the PCM to the mount. Slide the PCM out of the mount.
Slide the new PCM into place. Replace the screws to secure the new PCM to the mount. Plug the securing bracket back into place to ensure the PCM is secured. Replace the mounting bolt using the 10mm socket.
Reinstall the glove box by sliding it back into place. Replace the three screws and tighten to secure the glove box to the vehicle. Close the glove box door. The PCM is now replaced and ready for use.