206,000km, 2009 Versa sedan, base model 1.6 manual. I've been getting code P0420 for a while, and I noticed that my fuel efficiency dropped by about 5mpg. A while back I also had an issue where it randomly went into limp mode and threw code P0171. I cleaned the mass airflow sensor and those...

I've been proud to own and do most of the work on my own vehicles. From basic maintenance to things like valve replacemenets. I've rebuilt motorcycle engines, I run a HVAC business, and I have a BS in physics. I'm just trying to show that I'm not some dunce when it comes to mechanics, engine...