Chevrolet: 2002 Malibu Service Engine Soon Light, dirty oil, computer malfunction

Question
For the past 2 months, the Service Engine Soon Light will periodically come on and remain on.  I had 2 mechanics check the car; both claim to find nothing wrong with the car, but the light keeps coming on.  On one occassion, several lights came and stayed on.  One mechanic even said that the sensors are noting mothing wrong.  Is this a simple computer malfunction, or may I have a potentially expensive problem?

Answer
did they look to see if the cas cap is or was off or loose  and dirty oil can do it also.If they say nothing is wrong I would not worry about it as long as the oil light  or hot  light  or  altanator light does NOT come on..it should be fine if they checked it but if any of those lights do come on seek help.