Code P0456, very small emissions leak

The gas cap is the most well-known cause of a leak, but it is only one of many possible causes. First check out this article:

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

This covers some of the things you can check yourself. You're mainly looking for rusted hose clamps and dry-rotted hoses. Those are things you can inspect visually. The location of the charcoal canister will be shown on the vacuum hose routing diagram on the emissions sticker under the hood.

For the more elusive causes, we use a smoke machine to inject a white, non-toxic smoke under two pounds of pressure into the system, then we search for where it is sneaking out. You might be able to borrow one of these from an auto parts store that rents or borrows tools.