My Niece has a 2001 Jeep Grand Cherokee, the " Malfunction Indicator Light" came on today, steady, not blinking, She just got gas, I checked the cap looks ok, checked other fluid levels, no issues.

Any guesses?

Stop at Autozone or one of the major parts chains.
They scan the computer codes for free.
See what code(s) is/are stored.
Possibly can be from fueling.
Maybe the fuel cap wasnt secured tightly..

Did she leave the vehicle running while fueling?
That can set a code too

Maybe something entirely different as well.
Get codes,see what they say and post back
Someone here should be able to assist

Jim