P0455 Code Help
 

Bought my 2001 S80T6 and within hours it threw a P0455. I already checked all of the vac houses and cannot find a leak anywhere. The car has once shown the "Check Fuel Cap" but not again since. Haven't checked the Evap canister hose but i don't think it is the problem.

Anything else I can check out before having a shop take a look at it?