This is related to the fuel evaporative system. Kia has a technical service bulletin (TSB) for this issue, have your repair shop look at TSB KT2005022406
Here is a brief description of the bulletin:
This bulletin provides information relating to diagnostic trouble code (DTC) P0446/P2422 for all Sedona, Sorento, Amanti and Optima (2.4L only) vehicles. The DTC P0446/P2422 sets when the fuel tank pressure sensor measures excessive vacuum in the fuel tank. P0446/P2422 is NOT related to a system leak.