Do you use the updated evap part gm provides?
https://downloads.innova.com/tsb-fil...00/2380966.pdf
#02-06-04-037F: DTC P0446 (Restricted/Blocked
EVAP Vent Path) Set, Check Engine Light On (Replace
Evaporative Emission (EVAP) Vent Valve Solenoid
Assembly and Add/Relocate Filter Box Using Service
Kit) - (Aug 5, 2009)
I just fixed this problem. I installed the updated evap part, a new gm fuel pump and the PO446 code was gone.