I had my check Engine light come on and I checked out with a AutoXray computer. It read a code of PO449 and I looked on the computer and fixed it myself. I wanted to post this to help anyone else who deals with this problem.
I first replaced the purge valve that is located on the front of the intake manifold under the hood. I was still getting the code after I reset it and had to look further and found that I have another valve located under the bed of the truck, located on the inner rear of the fuel tank attached on the outside of the tank. It is a black canister with a valve attached to it with a hose attached to it. I called the dealer and they want $288.00 to replace it with a kit that has a longer hose to move it in a safer area to avoid dust in the canister. I did try to just clean the canister and I made sure the hose was not clogged but still had the code problem. I did more research online and was able to just buy the vapor canister purge solenoid valve from am-autoparts.com for $30.50 with free shipping. All I did is just disconnect the wire connector and the quick release hose that is attached to another black plastic box assy that is located above the fuel tank. I had to install a new 4" rubber hose to attach to my new part and I reset code and it fixed the problem. There is a diagram of the part and fuel tank system I found on-line and the web-site is www.gmpartsgiant.com/components/2007-gmc-sierra-classic
. Just go to the Fuel,Exhaust,Carburetion,Emisssion. You do not need the service kit that they tell you you need and that part# is 19152349 for $153.80. Just get the part from am-autoparts web-site and that part#214-2149 and this will solve your problem. If you have any questions, please just E-mail me @ firstname.lastname@example.org