Hi guy's I'm going to shoot in the dark and place this here.
I have a 99 chevrolet Blazer 4x4 that I keep getting code p0420, I've looked through your forums for hours and didn't quite find anything that I could use. I did however, find a nice evap schematic, but I need to find something more specific.

Problem: about 8 months ago I had to replace my cat because it was rattling (yeah it was an annoyance than anything I should've left it alone) But anyway I bought a Magnaflow Universal and things where going good untill 4 months ago when my SES came on. I checked the code p0420 it told me Catalyst System Efficiency low Bank 1. So I cleared the code and all was fine untill about five or so miles down the road. It came back on again. Anyway long story short I replaces the after cat O2 and this still didn't fix my problem. Is there anything I can do to fix. Obviously I should change the cat or is there a recalibrated O2 for an aftermarket catalytic converter.
If I do have to replace the cat where would I find a OEM Cat?

 

you had a cat rattling? i have a 98' 4x4 that has the same problem.

any clue what it was?

 

Yeah, it was a peice of the honeycomb that had broke off.

 

Sounds like your PCM doesn't like the replacement cat. This happens on some vehicles. If you have tail-pipe emissions testing, your replacement cat likely won't pass that kind of test. But if you do not, you could try a 'spark plug anti-fouler' fitting which will space the O2 sensor out of the exhaust stream enough to likely get rid of the code.

 

Ok thanx I will try that. Because I sure don't have the money for a new OEM spec Cat.