Hello people... new to this thread.

I have a 2006 GMC Sierra crew cab; 5.3L, Z71. About a year ago, it threw the P0446 code. I replaced the canister at that time. Since then, the CEL comes on about every 3 months with the same code. I would just clear the code and not worry about it. Now, it is time to get the bi-annual emissions test. I stopped by the GMC dealership and picked up another solenoid kit ($180 - ouch) and plan to put it on in the next few days. I assume the replacement solenoid I installed is bad but (being an electrical component) they won't take it back. I plan to blow out the line and replace the unit and canister. Is there anything else that I should do while under there?
Question: What does this silly solenoid do? Does it serve any useful purpose or is it just un-necessary complication? Seems to me that a simple check-valve to allow a little air in when needed as fuel is being drawn out would suffice. <Gripe off>

I have been a member of several other forums and appreciate the opportunity to learn from you guys. This thread has been particularly helpful

thanks,
- Jimmy