yep, downpipe plus cat, that's what I'm running.

 

I just spent the last couple of weeks working over my car in order to get it back on the road and when I scanned my car I had the same two codes. Here's what it turned out to be with mine.
First, the P4020 code. The source was initially a collapsed cat which I replaced with the ZZP one that you linked. This was about a year & a half ago. The code was cured for a little while. When it came back I replaced the rear O2 sensor with a new one and it stayed gone again till I took the car off of the road. (By then I noticed it was starting to overheat in traffic.)

Now after scanning the car in preparation to repair it I had two codes P0420 & P0480.

I cleared them & went after the P0480 one first. Checked fuses first, then relays & electrical connections. Finally the fans themselves. Turns out one of the fans was burned out. I replaced the entire fan shroud and that cured the prob there.

I drove the car for about 120 miles to make sure that the CEL stayed off, unfortunately it came back on. That was last Friday. P0420.

Called ZZP & told them that at this point it may be time to flash the PCM cause my car is a bitch! They said it was common with some cars for the CEL to go off with that cat even though its OK. I saw no point at this time to waste money on a emulator as I was getting more for the money with the PCM flash. I am now currently waiting on its safe return.

Hope this helps in some way.
Peace out.

 

Yes, when you replace the stock cat with the aftermarket high-flow cat there's like a 90% chance that there will be a P0420 code still, because the cat is working too well that the car thinks it's broken. The car is used to the stupid restrictive cat.

 

alright well thanks for the help guys, I really appreciate it it, I just ordered the new down pipe and CAT, and im gonna try to use the car as little as possible until then. I've heard that the P0480 can be caused by the fan wires grounding or a bad connection, so before i replace anything else im going to check all the wiring. I'll let you know how it all goes

 

OK my downpipe and cat came in today, but i was asleep so i didnt get to put it on, first thing in the mornin im gonna throw it on some jackstands and get started, you guys got any advice for me?

 

Chances are some of the bolts will be rusty. If you can/have time, spray them with liquid wrench or pb blaster. Have some 6 point sockets and extensions and maybe a torch. And hopefully, you won't need band aids for cut knuckles! It all should be pretty straight forward. I'm not sure what others might think of this, but I did invest in a O2 sensor socket and I haven't regretted owning it.

 

ok 2 the bolts that attach the donwpipe to the exhaust manifold... how the hell are they supposed to be removed, ive been out here for an hour tryin to get at em from every imaginable way and i cant do it....

 

Rock the engine forward?

 

i was afraid of that... I've tried gettin em at every angle possible, they been soaking in wd 40 for about 2 hours now, tried tappin em a lightly with a hammer to help break em loose. When u say rock it forward am i gonna have to remove the dog bones and pull it forward?

 

That's all you do.

Very easy. But you'll want to open the driver's door when you take off the dog bones, rock the car back and forth and watch the engine closely. When it gets to where you want it to be, step on the parking brake and it will hold there.

Simply remove the parking brake to let the engine go back to where it belongs.