What are problems that cause Gascap error code to come on? on 2006 Chrysler Pacifica

Originally, the check engine light was on. Had smoke vacuum test, was told it was the gas cap. New gas cap ordered from Chrysler dealer. Next week Gascap error came on, so I took it back. Once the light randomly went off and stayed out for several days then came back on. This was all on the same tank of gas, so the cap had not been taken off. So, I took it back for another smoke test and they can't find the problem. (Had catalytic converter replaced at dealer in 2011 thankfully was under warranty!) Current miles-102,000.

Asked by for the 2006 Chrysler Pacifica
Just had this very same issue & problem was fixed by installing new radiator!