Intermittent check engine light

My friend has a '96 grand Cherokee and the last couple of times she comes back across the mountains after visiting her beau the check engine light has come on. Both times it has gone out with a day or two and has not had any change in drivability. I've been trying to figure out what's up for her and I'm drawing a blank. Does anyone have any thoughts?

in a 96, the code should be stored in the ecm whether the light is on or not. what engine does it have, mileage, service history. the more info you give, the better answers you will get