1995 service engine soon light comes on after driving about 30 minutes. Why?

something to do with the fuel and exhaust as it passes the o2 sensor if on and off at times, you may try another grade or brand of fuel, or maybe a failing o2 sensor, many other possibility's that's most probity.

Here's where modern day mechanics have you by the .............,Your better of just paying what I call the rip off fee :) of about $80.00 for them to hook your car up to a machine to diagnose your problem (even the url above comes at a cost) in the good old days the mechanic was trained to know by prognosis what's wrong with your car and didn't charge you for something he should already know. probably not helpful but to the point

I have this same problem along with smoke coming from under the hood. Any ideas?