So my CPO warrantee just expired and at 100,400 miles I got these two codes, 4d16 and 46a4, and a CEL. I reset them and the codes stayed away for about a week. All the reading I have done points to either a dirty metering valve or a faulty aft NOx sensor which appears to be a $601.00 part and gamble at this point. Has anyone had this specific set of codes with a suggested way ahead?

Thank you in advance.

-Jeff