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

The check engine light on a 2002 Ford Focus staying on after an oil change almost certainly means the oil change itself didn't cause the light to come on. The light indicates a problem detected by the car's onboard diagnostic system (OBD-II) and unrelated to the oil change. The oil change mechanic might have accidentally left the light on after performing some other diagnostic work or clearing a code, or it was already on before the service.

Here's what you should do:

* Don't ignore it: A persistent check engine light means there's a problem that needs attention. Driving with a malfunctioning system can cause further damage to your car.

* Get the code read: You need to get the diagnostic trouble code (DTC) read from the car's computer. This can be done at:

* Auto parts stores: Many auto parts stores (like AutoZone, Advance Auto Parts, etc.) offer free OBD-II code reading services. They can tell you what the code means, which can give you a clue to the problem.

* Mechanic: A mechanic can read the code and diagnose the problem more thoroughly.

* Don't just reset the light: Simply clearing the code without fixing the underlying problem will only temporarily turn off the light. The problem will likely return, and could worsen.

The light coming on *after* the oil change is purely coincidental. It's crucial to diagnose the underlying issue, not just the light itself.