Code PO456, slow EVAP leak in gas cap?

P0456 - EVAP Leak Monitor Small Leak Detected

This fault code refers to the entire evaporative emissions system. The gas cap is just one part of that. There's the fuel tank, hoses, charcoal canister, purge valve, and a lot of other parts involved with this system. The first step is to do a quick visual inspection for things like dry-rotted hoses and rusty hose clamps, but usually small leaks can be very elusive. We use a smoke machine to inject a white, non-toxic smoke at 2 psi into the system, then we watch for where it sneaks out.

This article will explain it better than I can:

https://www.2carpros.com/articles/evap-system-code-repair