Here is a tip i found on my online repair info give it a try and let me know it would explain both codes let me know what you find?

5.3L Check engine light on, code P0449 vent sol. circuit.

Found vent solenoid rotten, it sits kinda on top of spare towards front of truck, you can see a small amount of rust coming through the body of the actual solenoid on the vent.

Wish i had one of dem dars picture cameras to show you.

Have replaced quite a few of these on these trucks

Answered by saturntech9 (expert)
23,155 answers provided

Thanks, so what exactly am I changing out?

Response from atimuamigi
1 question asked

Your not changing anything yet your inspecting the vent solenoid like the tip says.Iam far away and cant see or test anything so thats your job let me know what you find.

Answered by saturntech9 (expert)
23,155 answers provided

I posted a diagram of where the solenoid is.

Answered by saturntech9 (expert)
23,155 answers provided

Thanks brother it was the vent solenoid..pretty rusty since its left out in the open at the bottom of the base by the spare tire... Found an online forum and it seems to be an issue with these models from chevvy-I wonder why? But thanks for all your help. I'll be posting more questions in the future.

Thank you,

V/R
07 Tahoe

Response from atimuamigi
1 question asked

Your welcome thats what were here for glad to hear you found the problem.They need to out it out of snow and salt reach so that doesnt happen.

Answered by saturntech9 (expert)
23,155 answers provided

I know right. Okay so what about the code p0455?? How do I fix that...unless if i fix code p0449 than it'll fix that too as well??

Response from atimuamigi
1 question asked

If the vent solenoid doesn't work properly it can set a P0455 thats what closes off the system so it can test for leaks.So lets fix the P0449 clear the codes and see what happens.Let me know what happens.

Answered by saturntech9 (expert)
23,155 answers provided