Hey BC, This should get you heading in the right direction.
Usually this P0456 code is caused by an incorrect or faulty gas cap. Filling the fuel tank with the engine running could conceivable cause this code as well or if the cap wasn't properly tightened. Any of the following could also be the cause:
* A small leak in any of the EVAP hoses or fuel tank hoses
* A small leak in the purge valve or vent valve
* The EVAP Canister may be leaking