06 5.3 Silverado LT3 showing P0446 Code
 

I have an 06 Silverado LT3 Crew Cab with 5.3L Flex engine with 74,800 miles. Couple of weeks ago CEL came on. It had been almost a month since I got gas. Tank was about half full. Took to Advance Auto and they checked code. Only code showing was P0446. Had car checked at Auto Zone. P0446 only. I disassembled and cleaned gas cap as best i could. Back in 2011 the CEL came on and showed codes P0449 and P0455. At that time after research I replaced the Vapor Canister Purge Solenoid Valve. P/N 25932571. I think it failed as I had a bad habit of topping off with gas at pump. Don't do that any longer.
I think P0446 indicates a restriction. I have gotten under truck and looked around and solenoid I replaced in 2011 nor the charcoal cannister and all the lines to and from I don't see anything that appears loose or leaking. Before i take to dealer, is there anything else that is a likely cause for P0446 ONLY?
I've heard the purge valve under the vortec cover on engine can go bad. I don't drive the truck in dusty environments.
Should I disconnect vent lines from cannister, remove from vehicle and try to blow out? The round part of the solenoid I replaced - doesn't it have a filter in the round portion. Should I remove solenoid and try to clean. I don't have a scan tool.
Any other possible causes as i don't want to take to dealer and have him start replacing parts without identifying what part has failed.
Thanks
Mike in alabama