This code has set 3 times in the last month.Every time I do a freeway run and come to a stop at the bottom of a ramp. Just before the code sets I get a really strong raw fuel smell in the vechical. But the engine never even stumbles idles smooth ,RPMs stay the same. The only work that has been done it it was 3 months ago. I changed the lower and upper intake ,fuel spyder and distributor . Any ideas?

 

Administrator swartlkk breaks it down for ya here:
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
https://blazerforum.com/forum/diagno...plained-19820/

 

any po420 I have ever seen had two replace converters

 

There are several things that can set P0420, it's not limited to just the converter. Follow the steps in the link in post #2 and you'll find the problem.