Decision Making on P0420 and P0430 codes on Gen 2 2006 145k miles
I have had these codes for about 3 years now. I've somehow managed to pass inspection by clearing them using an ODB2 reader, but failed today. I'm tired of dealing with these codes and want to go about this the right way.
What decision making tree should be followed with these codes? I've scoured the Internet and while there is a lot of info on these codes on the Xterra, there is nothing that puts it together in a comprehensive manner that I can act upon.
My situation is the truck runs fine. Replaced spark plugs 2 years ago. Still have that timing chain whine which these trucks suffer from. I'm eying some cats at RockAuto, but before I spend the cash, I want to make sure I approach this problem in the proper way - the way an expert, honest mechanic would.