Hyundai Repair: scan code PO446 EVAP emissions vent control curcuit, charcoal canister, evaporative emissions

Question
What does that code mean?  What could be wrong and is it expensive to fix?

Answer
P0446 indicates there's excessive vacuum in the fuel tank/evaporative emissions control system.  In most cases, significant diagnosis is required to find the cause.  There are four basic types of problems that I've seen cause this code:

1.  Faulty fuel tank pressure sensor, causing the ECM to believe there's vacuum present when in actuality there is none.

2.  Purge control valve stuck open, applying vacuum to tank at all times.

3.  Blockage preventing the tank from properly venting.  This could be due to a plugged charcoal canister or canister filter, a stuck closed canister close valve, or any sort of blockage in the evaporative system.

4.  Liquid fuel blocking one of the vapor hose to the fuel tank pressure sensor, preventing the sensor from properly reading the pressure in the tank.