Pontiac

I went to auto zone and they checked it for me and told me the code was obd ii po442 they told me all i had to do was change the gas cap, boy were they full of it, can anyone tell me what could be wronge, thanks i replaced the gas cap 2 weeks ago and the light is still on, can anyone tell me what is all involved with the evaporative emission control system, also before i changed the gas cap 2 weeks ago when i took it to the e-check station e thing passed, even the gas cap, the check engine light was reset so they couldnt read the computer so they just did a run on the car and e thing passed,

A gas cap that doesen`t seal will set this code, but so will any sort of pressure loss in the gas tank or related components that have to do with preventing vapors from escaping to atmosphere. the best thing to do is to have a dealer run a smoke test on the evap control system,to exactly pinpoint leak.