1996 Dodge Intrepid no explanation

Smells problem
1996 Dodge Intrepid 6 cyl Front Wheel Drive Automatic

my moms car had a bad battery in it; before I changed the battery thier was a check engin light on
and the smell of rotten eggs.
after I changed the battery in the car the light is now off and the smell is gone.
i know that typicaly the light will come on again but has not, and the smell is gone.
anyone have any idea for this strange occourance.