service engine soon light comes On in Ford 2002 Explorer?

the service engine soon light is not for the oil or anything like it, it is warning u that the electronic engine control system has a fault present, there are about 200 poss reasons so you will need to get the system teasted, Kragen and Autozone both do this test for free, just ask for a OBD 2 fault code test, don't listen to the suggestions of what is wrong, they are not trained in this and often you end up buying parts that don't fix the problem, seek a Ford dealer out or other trained electronic engine control tech for assistance in fixing the problem.