Magna-flow cat
 

Does anybody else have a high flow Magna cat on their truck thats causing a P0420 ?

I do not, but was told that if you replace the cat with anything other then one from GM it will consistently throw that code. Hope this helps.

well, I don't know, it's been on there for about five months now and only came on once. I think I may have a CPI problem, because its the original unit on the car. I peaked through the hole in the top and saw a big clean spot.....

Its not, a problem wih the cpi, speaking from a GM mechanic. Just because it has to throw a pending code now and again it will trigger the SES light. The cat is fine (as long as its new) but the O2 sensor just has a problem(as in it doesn't recognize the flow to the GM specs).