Knowing from reading that the P0446 code indicated a blockage and is sometimes difficult to diagnose, here is how I happened to fix mine.
Having checked, disassembled and cleaned the gas cap and gasket (lubing with a tiny bit of silicone grease), I looked at the lines to and from the charcoal cannister. back in 2011, I replaced the evap solenoid by the charcoal canister as I was getting P0449 and P0455 codes. I think it failed because I tried to top off the tank. The replacement was the 25932571. It had a round body and connected to the cannister via one hose.
the canister had 3 hoses to it. I removed the evap solenoid and the canister from the vehicle and did the following.

1. tested the evap solenoid with 12 v to verify it opened and closed. It did.
2. I removed the small coarse black donut shaped foam filter from the solenoid and washed it in warm water, dried, and reassembled after blowing dust and wiping out the filter location in the solenoid.
3. I wiped the inside of the female hose fitting on the solenoid and applied a tiny bit of silicone grease to the fitting o-ring.
4. On the canister I wiped the 3 male fittings on the end of the canister clean and applied a tiny bit of grease all around the 3 fittings.
5. i wiped clean the inside of the other 2 hoses going to the canister and applied a tiny bit of grease to the o-rings.
6. I reassembled everything.
7. Started the truck and I had driven about 3 miles when the CEL went off.

I can only assume that some dirt or something was allowing a little bit of vacuum to escape during the evap test and my cleaning and lubing o-rings closed that leak path. At least for now, no CEL. I plan to take it by Autrozone and have them check to see if the P0446 code has cleared.
Maybe this info will help someone else.
Mike in Alabama

 

Thanks for taking the time to let us know how you fixed this one. So many never come back. Good luck, I hope it stays fixed.

 

Happy to pass along what worked for me. We will see if it stays fixed. from what i have read that code is difficult to pinpoint the cause of so i was just trying anything to see if it worked.
Mike