2005 xc70 engine light code #PO456

[breedencm]

I am an owner of a 2005 xc70 and the check engine light keeps coming on. (codePO456). I normally disconnect the battery to reset it, but it keeps coming on. I have bought/replaced the gas cap and ensure it is replaced each time securely. I realize that this seems to be be a common subject, but was wondering what else I should check?

Thanks for your input,