i use to change the oil for my girlfriend's car and her mom's van until one day when taking the car out after a fresh oil change, the CEL light came on and my gf freaked out. She blamed me for it coming on and didnt want me near her car. I asked her if she pumped gas that day and she said her mom had put gas in it right after I had changed the oil. Lo and behold, the gas cap wasnt tightened at all, thus throwin a CEL. After that, I told her she could go pay the $35 at the local oil change place if she wanted her oil changed.