03 GMC Code P0449 = Bad Gas mileage Help!!!

[jeepjones85]

Hi I have a 03 GMC 4x4 ext cab And my check engine light came
on So i got my code reader out. It is a Can OBD2 Reader from
O'Reillys It said '' P0449 " In the book is said ,Evaporative Emission
Control System Vent/Valve/Solenoid Circuit Malfunction
I called all the parts stores and they said call the dealer that they
couldn't help. Maby somebody can tell me what this it Because
when the light came on I started to get 16mpg And before
the light came on i was gettin 23mpg Thanks For Any Help