'07 Suburban p0455 but "no" leak

[mag68]

I have a 2007 Suburban that is a 5.3L flex fuel with active fuel management. The check engine light has come on with code p0455, "mass evap system leak". I took it to a shop and they ran a smoke/pressure test with the motor hot and cold. It passed both tests. They replaced and moved the vapor canister vent solenoid, but that didn't help. I'm about to sell this vehicle and wanted to fix this issue for the prospective buyer. Are there any cheap, and relatively easy ways to diagnose the bad component that is causing this code? I don't want to spend lots of money fixing the problem, but also don't want to try and sell a car that has a check engine light on. Any pointers on how to trouble shoot this would be appreciated!