Need suggestions on fixing service engine code P0455 on my QX56. Already replaced gas cap and cleared code for it to come back again. Which of the two sensors should be replaced first?

Welcome to NICO Club!

The procedure for code P0455 will be in the EC chapter of the FSM. You don't share the year of your vehicle (significant information). Here's a link to all of them:
http://www.nicoclub.com/archives/infini ... nuals.html

You might just need a hose, relief valve or vent valve. The diagnoses will help you determine the correct parts to buy. Much of the testing can be done without a consult computer, just read through it all and you'll see what you can do with your own tools. Please let us know how it goes of if you have detailed questions about any of it. Good luck!