Our 2011 Ford Fiesta S 1.6L I4 check engine light came on & OED is giving a P0420P code. What would be best thing to start with checking? We haven't had the car very long at all & wanted to ask before I take it to a mechanic in case it's possibly could be something easy to fix that we can actually do ourselves. But if it's possibly multiple things that could be the issue & will need to be checked I'll prolly just take it to shop instead. Just hoping someone can tell me anything to narrow it down hopefully at least so I know what to expect them to say anyway. Thanks for any help with this issue.