A lot of times this comes up because the computer has detected an evaporative system leak. Make sure the gas cap is tight and if the message still comes up you can try a new gas cap. If still a problem you probably have a leak in the Evap system which will require repair. Even though you think your gas cap might be ok buy a new one it probally has a pressure release in it and it is no good. After replacing the gas cap then disconnect the positive cable from your battery for about 10 seconds and put it back on this will reset the computer and should take that message away. Had this problem, needed to diconnect both terminals for the code to clear. Thanks for the help in resolving this problem as GM dealer wanted $$$$$$ to clear code which only takes a minute with portable computer they have. No wonder they are losing market share!!!!