code po449: need help

I have this po449 code. Changed gas cap already and looked everything over. Any ideas????
2003 sierra 5.3 T ....

Check this link. http://service.gm.com/gmspo/mode6/pdf/1999/99t57R_LD_tE.pdf, this requires tracking down vent solenoid issues, I guessed at you trucks year as you did not post it ( must)