Engine light 2003 H2 Help...

Hi,
 
New to the forum and am having a problem I can't seem to tackle. Engine light came on a few weeks ago and I took it into Autozone to have the code read, Which was PO455
 
At the time they said it looked like a gas cap issue and mine was not tightning as it sould so I replaced it, they reset the light and said here's hopping it stays off! Well I was happy cause it stayed of for almost a week, BUT today she popped back on,... I stopped back by to get another code read and was told it was the same code, but now with the cap replaced it was saying a large leak in the evap system which could be evap can broke, hose cracked or fell off, or purge or vent selinoid defective, or... vacum leak at engine.  I feel stupid but I have no idea where the Evap canister is, and could find nothing pointing me at it.  I don't think its a Vac leak at the engine cause the motor runs perfect, not a miss... not a sputter...  So... if any one of you really kind people can help with the code,   first place you'd look ect.    Actually a pic of the evac canister might help.... 
 
Thanks in advance....

I had the vent solenoid replaced for p0446 and also the cannister for p0446...
 
I think p0445 is for the relief valve on the top of your gas tank...
 
 

The canister is located on the drivers side frame, about mid vehicle.
 
 

The 455 code is a large leak in the system. If it is not the cap, then a line or hose has to be off or cracked. Or it could be a purge valve stuck open.
 
The H2 runs a check in between 1/4 and 3/4 tank of gas. That may explain why the light did not come back on right away. If you keep your tank full or above 3/4 it will not tag the light. As soon as it drops below that, it runs a check on the system.
 
Check all the vent lines to make sure one has not come off.

Good info here.

Thanks Blackstang,
 
I located the canister and crawed all around it...  the can looked like new... none of the three hoses off or loose One large and two smaller.  I evenlistened cause you'd think with a magor leak one would hear the sucking sound?  But nope...  crawled all over the engine...  could find not obvious leaks, cracks... ect.  Im stumpted cause Im not a silly, stupid man in my prime I even built a few racing engines.. so this is actually rediculas!  GRrrr.. Besides the 100 dollar bill my hummer dealer wants just to run a diagnostic! They are like 60 miles away.. Im in construction and we work daylight till dark so I'll need to rent a car, just to drop my beast off...  I mean she's worth every dime but dang it just seems....  

so... while you were under there did you check the relief valve on the tank?

this should help you out on solving your problem... http://service.gm.com/servlets/BlobShtml?ShtmlFile=1251046&psdid=732&evc=sm

that link doesnt work for me.....
 

I couldn't get the link to work either.

sorry it might only be accessable for technicians... ill try to get something up for you...