Hyundai Repair: diagnostic code p0446, emission control system, purge valve

Question
Evap. Emission Control System-Vent Control Malfunction p0446.
What needs to be fixed when a diagnostic gives this error code.

Answer
Unfortunately, there are a large number of problems that could cause this diagnostic code.  The code indicates that vacuum is retained in the fuel tank.  There are three basic types of failures that cause this code:
1.  Fuel tank pressure sensor reads too low causing ECM to think vacuum is present in tank when in actuality it is not.
2.  Purge valve stuck open, applying constant vacuum to fuel tank.
3.  Blockage preventing fresh air from being sucked in through the canister.