05 Dakota 3.9l auto. i have a recurring problem with P0456 and P0457 codes. system was smoke tested from the purge sol. to the nvld on canister with the vent hose plugged. originally the nvld sol. was leaking at the gasket and at the connector area. changed the nvld sol. and still has these 2 codes coming back. re smoked the whole system and holds pressure and ball will drop all the way to the bottom of gauge on smoke machine. run forced nvld test with scan tool and fails nvld portion of test.

 

Have a read here. See if it helps at all.