Service engine light is on most of the time. Tech finds nothing wrong. on 2002 GMC Sierra 1500

Service technicians have checked repeatedly and find nothing wrong.

2 answers
if the light is on there has to be a code.poss need a better scanner.some of the smaller scanners can't read all data
My car was doing this--turned out, it was the gas cap not seating well--cap was not turned enough times to tighten. (You may have to turn off ignition/turn on/turn off a few times for the light to go off if the cap is the issue).