P0455 is a pain sometimes because you have a leak in a purge valve or a rubber fuel vapor hose, etc., and you will need to identify all the parts of the system, and then trace each one of them in turn.
BTW, as an aside, I tested when the DTC's clear & set by removing my gas cap for six months as described in this thread:
- What happens with the P0455 diagnostic trouble code when the gas cap is missing (1)
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