Check Engine Light Keeps coming on

My CEL came on 2 weeks ago while driving on the highway. I had a guy @ autozone run a diagnostic with the cumputer and came up with the code: PO440 = Evaporative Emission control system malfunction
He told me that the most common cause was a loose or faulty gas cap so I bought a new one and he cleared the code for me. Then I filled my car up full w/gas to test the new cap and no CEL came on until 2 days later. Now its been on for about a week and I dont think that its the gas cap. What else/where should I look for that could be a possible cause?

I have the exact same problem. Same code, and same answers from Checker, and Autozone. We never got the Gas cap fixed because they told us that MIGHT be the problem and you knew that these guys didn't know what they were talking about. I have no clue where to start to fix it so I just leave it on. Doesn't really bother me anymore. :)

Yup. I had the same problem on my 01' Cavi. Took it to the dealer about 20k mi ago. They replaced the gas cap. Said to make sure it's tightened. The CEL hasn't been on since...

It could be a canister malfunction....This canister look like a big black rectangular can....Its control the fuel vapor emission....Scuse me for my english....