Do you happen to have a "p-code"? (I think OnStar can provide this)
Ok- P0442 is "EVAP (emissions) System Small Leak Detected"
This is actually a very tough one to diagnose. Seeing as we are looking for a "small leak", it's almost impossible to see it "by eye" or to say "This is what has the leak". The best, XXXXX XXXXX way to diagnose this problem is to have the EVAP system "smoked". This is where a machine forces smoke through the EVAP system, and the leak will reveal itself with a stream of smoke. Most shops charge around $40-$65 for this service, but it's money well spent, as you'll drive yourself nuts trying to find a small leak in the entire EVAP system.