2003 Chevy Impala CHECK ENGINE LIGHT

Engine Mechanical problem
2003 Chevy Impala 6 cyl Automatic

check engine light is on. Took it to service center I have dealt with since warranty expired. 91,000 mile, serviced every 3500 miles. As computer check stated, he replaced oil pressure switch and fan relays. I replaced gas cap. Light still on. He check transmission, exhaust and etc. No problems. Mechanic tells me to drive it until something shows up. Would trade it in, but no one wants it with the "check engine" light on. Ant ideas.