The "Check engine" light in a Ford Focus is designed to stay on until it has been deactivated by a mechanic at an auto garage. If the mechanic forgets to disable the light, or if you prefer to make repairs yourself, you will need to deactivate the light on your own. This can be accomplished with an Onboard Diagnostic Code Scanner, which is available from several manufacturers.
Locate the diagnostic input jack. It is below the steering column, either in the centre or right side of the dashboard panel.
Open the panel covering the input jack and plug in the scanner.
Turn on the scanner if it does not power up automatically.
Turn on the car's engine.
Observe the scanner's screen for the car's trouble codes.
Clear the trouble codes with the scanner, either one by one or all at once with a "clear" button (if available). Your "Check engine" light will turn off when these codes have been cleared.
Turn off and unplug the scanner and replace the input jack cover.