I have just replaced my air mass meter to get rid of the cel for the p0171 code when 2 weeks later my p0420 code pops up!

The thing is i am getting better gas mileage than ever now ranging from 20-22mpg. So i am curious to what you guys think is throwing the code? should i replace my 02 sensor(s), or my cat, or all?

Its a 2000 3.4 5spd non cali

Thanks

 

P0420 is usually the rear cat sensor (bank 1, sensor 2), check out the link.

As for solutions, I'm running a URD simulator, which got rid of that CEL code for me.