1997 E-150 code 0446
 

I have a 97 E-150 with a 4.6L but I keep getting 0446 code (Evaporative Emmission Control System Vent Control Circuit) What should I start with? I know there is a vacuum switch is used to detect when flow exists. But where is it? and could that be the problem? Also where is the EVAP canister on this van?