99 GMC Sierra 4.3 V-6 110,000Miles - OBDII Code PO446

So, Ive been having nothing since headaches with my truck since January, when my smog and registration were due. first it was the PO300, got that squared away thanks to a few guys on here. So, I've finally decided to post my new problem...
 
PO446-Evaporative Emission System Vent Control Circuit. 
It's coming up twice.

 
The things i have all ready tried doing are-
-Replace gas cap
-Replace the vent valve (which GM has now rebuilt and turned a $50.00 part into a $200.00 part. Thanks Guys!!!)
-Replaced the charcoal canister
 
I thought for sure i would have eliminated the problem but nooooo...
 
Truck runs great but i cant get it smogged. Any sort of help would be greatly appreciated. This is my daily so it being down for so long has really put a damper on me and my family. Granted we have another vehicle but the wife and i are playing musical town car almost every day, switching back and forth between lunches...
 
Once again, I appreciate any help!!!
 
Thanks Fellas!!! 

Get a good engine scanner. Say Snap On Gensis, etc. See if the leak detection pump or evap purge solenoid is getting its proper reference from the ECM. You are replacing parts not the problem. If it was the items you listed it would read P0440 or P0441 evap leak large/small. It is telling that the circuit is broken. Check your reference and voltage to the evap solenoid, leak pump, and purge solenoid.  Hope this helps.

damn mater you f'n rock bro...i appreciate the help as ive been bashing my head against the wall and as most mini truckers (im assuming) i get all depressed and shit when my trucks down...lol..not suicidal or any thing...just a little mopey l...how would i go about checkin the voltage and things of this nature? ive gotta super nice fluke meter but ive only done commercial electrical work. i wouldn't know where to even begin or know the proper readings. would you mind explaining for me please??? once again, i cant thank you enough for your help...    

Bump...

You can do this with a regular volt meter as long as it reads DC. Get a wiring diagram for your emission circuit and go through it. All of the sensors should have a 12v switched, ground and a 5v reference wire that reads to the ECM. Turn the key to run and check the voltage and refernce voltage without the truck running. Your refernce voltage should be in the .5 to 1 vdc range - without the engine running - they should begin to fluctuate once the engine is fired. That is normal. If all of your readings are in range then move to a scanner that can see the ECM with the truck running. Go into the presets and look at the sensors in the emissions circuit and see if any of those are throwing the ECM out of a whack. A good Snap-On or Gen 3 scanner can do this. As said before, your ECM is telling you that you have a circuit problem, not a vaccum leak. Hope it helps.

nice...i will do that. i certainly do appreciate all your help and patients. i will keep you posted with the results!!!   thanks again!!! 

so, i've taken the truck to two different mechanics...they cant find the problem either...they've checked all the electrical and evap...what should i do now??? i have a feeling i'm screwed...

bump...

bump...

bumps....