check engine light

Can anyone out there help me? We own a 1998 Mercury Mystique. The

"check engine" light has been on for so long now I can't remember when

it was off last. It would go off for a few days and back on. Our local

Mercury dealer has looked at it on about 4 occasions The last time it

cost me $160,00. It was off for 5 days and back on. They have all but

told me to live with it. Anyone know what to do?