Hyundai Repair: Code p0446, emission control system, fuel vapors

Question
I had my check engine light checked the code that came up was P0446 Emission Control System Vent Circuit... I understand that it might have to do with the gas cap but the turning of the cap doesnt help, what I did realize is that when I take the gas Cap off it makes a sound like its releasing air?  

Answer
An incorrectly installed or malfunctioning fuel cap has nothing to do with code P0446.  This code means that you've got vacuum building up in the fuel tank/evaporative system.

The possible modes of failure are as follows:
1.  The fuel tank pressure sensor is reading low, making the computer think vacuum is present when it is not.  (This is not likely, since you hear the air sucking when you remove the cap).
2.  The purge control valve is stuck open, applying engine vacuum at a faster rate than it can vent.  (This is also not likely, since turning the vehicle off should give it time to vent).
3.  There is a blockage or restriction between the fuel tank and the eventual vent through the charcoal canister and the canister filter, causing vacuum to build up when the ECM attempts to purge fuel vapors.