What does it mean if the check engine light of a 2002 Ford Focus is still on after having the oil changed?

Go to auto zone and have the codes read for free. They will also look up the meaning of the code for you, but be careful about taking their advice on how to fix the problem. If you have an older Focus, buy a code reader on eBay for approx. $50.00 USD. (OBD-II) because you will need it a lot. The Check Engine light is always related to something in the emissions control system, e.g., a bad sensor, a leaky vacuum hose, etc. Other auto parts stores may also read the codes from the computer for free. Do not pay a mechanic just to read the code. You can try clearing the code from the computer and waiting to see if it comes back. The computer connection port is below the steering wheel, to the left, behind a little plastic cover.

Answer

Autozone is NOT the answer.

Pull the codes from the computer, match the code to the troubleshooting procedure, follow the procedure to find the source. Repair the source, light will go out if that was the only problem. There are "monitors" or self tests the computer runs the car through a drive cycle, if a problem occurs, it may not run all of the self tests until that problem is taken care. Therefore, another problem may exist. It is emission related. OR hook up a scanner that is capable of clearing codes, and hope that none are still active.

Answer

Mine was a oxygen sensor