P4020 What else could it be?

[my1stss]

Ok when i got my car last May the cats were clogged so i put new magnaflow cats on and changed all the O2 sensors, the code remained after several others for ehaust. So i just had the check valves replaced both of them and the service light finally went off so i thought i was all clear. Of course two weeks later the engine light comes on again its P4020 or P0420 now i cant remember damn it, what else could it be. Is there a crazy chance that the cats are bad again already? Please help i am at my wits end with this engine light i have to have it right.....Mama Steph!

[2001NBMZ28]

Think the code is P0420? That's for the drivers side cat. The check valves with throw P1415 (drivers side) or P1416 (passenger side) - if both fail it'll throw something else. The rear 02's are what monitor the cats. Aftermarket cats are probably marginal if "high flow" to begin with. Make sure you don't have an exhaust leak near the 02 on that side.

[my1stss]

Ok so what should i have the man look for? Oh and i had a new smog pump put on too. Im sorry i dont mean to come off rude, but its easier for me to say can you check here to see if theres a leak than to say check and see if theres a leak, it seems when i give him ideas it gets done faster lol.

[my1stss]

I think i understand you mean check for a hole in exhaust pipe, or near welds maybe? I try to understand the best i can but i have never been under a car lol so i do the best i can with what i read and can see.

[my1stss]

I wonder if because we did the O2's with the cats and didnt do the smog pump and check valves until months later if it couldve possibly fouled out the O2's???

[2001NBMZ28]

Just look for an exhaust leak near the drivers side 02 post cat sensor. You're not getting any secondary air codes now so water under the bridge. IMO there was no need to replace all 4 02's just for air or rear 02 codes...

[blue02Z]

could be the use of aftermarket cats on the stock tune. easiest way to get rid of it and some more hp is to send your pcm to Frost and have him tune out the rear o2 sensors.