The power train control module, or engine computer, on a Ford Focus runs periodic tests on the various engine management systems while the Focus is running. If the module notes any variations in the data returned from these tests that does not correlate with factory preset data, it generates a diagnostic trouble code. Once the module generates a code, it then turns on the malfunction indicator lamp, commonly known as a check engine light. Once you've retrieved the codes, made the diagnosis and corrected the problem, you'll need to reset the computer. Anyone with basic auto-repair skill can do this.
Sit in the driver's seat and remove the dust cover from the diagnostic port located just below the dashboard between the steering column and the centre console.
Plug the scan tool's datalink cable into the diagnostic port. Turn the ignition to the run position.
Follow your scan tool's specific operating instructions to perform a quick test on the power train control module.
When prompted to erase the diagnostic trouble codes, do so.
Complete the quick test. Disconnect the datalink cable from the port and place the dust cap over the port.
Different scan tools have slightly different operating procedures, but commonly the tool will walk you through the quick test by prompting you on the tool's LED screen to make selections with the tool's keypad. You can purchase a scan tool at most automotive part retailers. As a Focus owner, make sure before you buy one that it functions with Ford OBD-II software.