Evap System 03 Impala

Does anybody know how the EVAP System works on a 03 Impala, or what the major components are? I read the thread on EVAP solenoid and I was able to test the solenoid on my car and it seems to work (open and closes with 12volts). The reason I ask this question is because I was trying to get my car inspected and the EVAP system is failing or not passable. What does the computer test on the car to determine if the EVAP System is passable or not?

If your gas cap is defective or not tightened sufficiently , you will get a code indicating an Evap system problem . I don't know if this is your situation but check your gas cap to see if it is tight ( three clicks when putting it back on ) or see if the rubber gasket is cracked . A new cap isn't very expensive and might solve your problem ? Good luck and keep us updated !