Having trouble figuring out my P0420 code. A little history... It is a 2008 suburban 1500 with the 5.3 flex and afm. I bought the truck a couple months ago. Shortly after purchasing, it had a collapsed lifter. I replaced ALL the AFM lifters. After it was all put back together it had a P0420 code. I replaced the Y-pipe with both cats, put new upstream Denso o2 sensors, all new AC Delco plugs, cleaned the MAF, cleaned the throttle body, replaced the air filter and ran a can of sea foam through the gas tank.


Now it still has a P0420 code and seems to idle rough when its warm. I have noticed that if I clear the code, it wont come back on until the EVAP check is done.


Is there something in the evap system that would trigger the 0420 code and cause it to run rough? There are no other codes or symptoms. I am stumped here.

 

what kind of cats did you put on? were they universal cats?

 

They are aftermarket cats, but not universal. I had to replace the whole y pipe with both cats as one unit.