I have been having a really hard time getting my 2001 blazer 2wd to pass emissions inspection! I HAD 8 codes when i started! In order i have fixed the ground wire for the secondary air pump then i fixed the fuel tank sensor next was the thermostat and ECT sensor then i replaced both upstream o2 sensors bank 1 sensor 1 and bank 2 sensor 1. I also done a tune up new rotor cap plugs wire's oil change and i cleared all the codes! I went though the gm cycle several times for 2 days but the secondary air pump or the catalyst would not go to READY then the cel light came back on for code p0420 this was not one of the original 8 codes and the cat was just replaced 1 year ago it only has about 40k miles on the cat! My REAL question is do you think it could be the downstream 02 sensor since it has probably never been changed and the truck has around 165k miles om it but it runs perfect I'm lost i sure hope that it's the downstream 02 sensor? PLEASE HELP!

 

Have you looked at this thread:

P0420 Explained