What would you do in my situation?
 

Okay, to begin with, I have to have emmisions where I live, so keep that in mind. Now, I am 16 and I just recently got into the import scene. Me (being young and narrow sited, bought a 2002 Civic LX that had already been given a few mods. They had taken a 2005 VTEC engine (D17A) and put it inside of it. Now it has a brand new CAT and 02 censors. The engine light came on about a week after the CAT and 02 censors were installed. The code is P0420, which means Catalyst performance below threshold. I have no idea what to do. Some people are telling me to keep it, others are telling me to sell it. Some people say its the ECU, others are saying its my racing headers (which I thinks makes no since). Everyone is telling me something different! We went and talked to a speed shop and they said they WONT EVEN touch it unless the person who made the mods is present, and they suggested I sell it. The problem is I love this car so much... Yes it has a very small engine, and I do want to drag race, but just like, once to see what it is like. I have no idea what to do... What would you do in my situation?

I wouldn't go back there.

As far as the P0420 goes, it's just a rationality check between the front and rear o2 sensors to determine if the converter is working at the accepted efficiency. If the rear o2 is giving a weird reading (weird as in just plain weird or weird as in "not matching the front sensor only on a smaller scale", it doesn't matter) for two consecutive driving cycles, it'll set the code.

So when you replaced the converter, what got replaced? Just the converter, rear o2, and exhaust gaskets? It could be an issue with the front 02 as well, keep that in mind. Are you using an OEM converter or aftermarket, and is it in the factory location?

I highly doubt it's a ECM (ECU) problem as I've seen enough cars set P0420s that I've lost count and not a single one of them was a ECM. Also, just so you know, the D17s don't take to mods well, even boost. If you eventually want to see some decent numbers you'll need to swap; K20s (RSX-S engine, 200bhp and respond to mods well) have come down in price in recent years and seem to be a pretty common swap on 7th gens.

Both 02 censors were replaced, and the CAT is aftermarket.

You may need a stock cat. I suggest fitting a stock EX exhaust system up to and including the cat, you can do whatever you want after the cat. It will never be a good race car without swapping the whole engine, so strive to make it a daily driver.

I think I suggested in one of your other threads to take the pipe off and look inside the cat to make sure it even has catalyst in it, i.e. it's not just hollow.

We have a 98 Jeep Grand Cherokee 5.9 Limited. It has a MagnaFlow Cat with a turbo muffler. Every so often it will produce a PO420 code. I have a reader & I just clear the code. Basically short of going back to stock from headers to tailpipe there is not much you can do. Just erase & move on. I know guys that have replaced both 02 sensors & still get that code. I have talked to a muffler shop & they basically said as I just did OEM the exhaust if it bothers you. I just live with it. Like Reaper said it is a reading between the two sensors Every so often they don't agree. If it is all the time I would start with OEM sensors (not aftermarket) & go from there.