Your 2002 Ford Focus's "check engine" light will stay on if you do not reset the trouble codes that caused the light to come on, even if you or your mechanic carried out the needed repair. Thankfully, clearing the codes and turning off the instrument panel warning light is relatively simply. All you need is an on-board dagnostic code scanner (an OBD II scanner). It can be the same one you used to retrieve the codes that led to the recent repairs. As a task, it will take less than five minutes.
Feel around the area beneath the Ford's dashboard and steering column. You will find a diagnostic input jack there.
Connect your diagnostic scanner to this jack. Some scanners will immediately turn themselves on if they sense a connection with the Ford Focus's diagnostic computer.
Turn the Ford Focus on. Try the electrical system first, and if a connection doesn't result, double check the scanner's connection to the diagnostic jack. As soon as a connection is realised, the Ford's trouble codes will appear on the scanner.
Triple-check to ensure that no new codes have appeared on the scanner. Once you are certain, press the "clear" button on the scanner. This will turn your "check engine" light off.