How familar are you we usimg the scan tools ? Have you used them successfully on a different vehicle ? There must be a code to turn on the light. Can you check the I/M readiness monitors with your tool, chances are after you reset the light one of the I/M monitors fails its test and then the light comes on. The EVAP is a typical one and does not run its test unless certian conditions are met, you can search foir the GM drive cycle which should get the system to run all its tests. Is the fuel gauge working properly the EVAP test needs an acurate fule gauge reading to function properly. If there is an Autozone near you have them see if they can get a code out, they will hook up for free and tell you what codes they get.