as seen on http://www.hyundai-forums.com/index.php?showtopic=46630&st=0

UPDATE!!! URGENT/MUST READ... Why am I getting the MIL/CEL? (CHECK ENGINE)

The secondary Catalytic Converter on the 2009 Hyundai Accent GS 1.6 IS NOT, I REPEAT, NOT A CAT AT ALL. There IS NO HONEY-COMB OR ANYTHING INSIDE THIS, IT'S A HOLLOW RESONATOR. IN ORDER TO FIX THIS YOU MUST PURCHASE A OBDII COMPLIANT CAT. YOU WILL PASS EMISSIONS AND EVERYTHING...

The part number I used as my CAT (PRIMARY CAT TO PASS ALL EPA TESTS) IS WALKER PART NO. #15053. The OD of the CAT is 2 9/16 and ID is 2 1/2. THIS IS EQUIVALENT TO THE SAME SIZE CAT ON YOUR EXHAUST MANIFOLD THAT YOU NOW CHANGED. I PURCHASED TWO FLANGES FROM NAPA PART NO. #31897 AND MIG WELDED THEM TO THE CAT. THIS NOW BOLTS ONTO MY CATBACK, AND THE DOWNPIPE FROM THE MEGAN RACING HEADER. NO CHECK ENGINE LIGHT, NO EXTENDING THE O2 SENSOR. VERY VERY GOOD. Now you have the performance of a header, with no illegal emissions, and actually better wtq. because of the newly created back pressure.

Difficulty Out Of Five(5): Two(2)-Three(3)
Notes: Pretty easy install; 99% Bolt-on. You need to drill the holes on the end that bolts to the cat bigger so it'll snug, and I HIGHLY recommend getting new grade-8 nuts and bolts. FOUR OF THEM. You only need two though if you're tight on cash I don't know...

Tools: Big drill bits, socket set (up to 22mm) and a impact gun or air ratchet for fast installation. Everything was really easy. A jack and two jack stands, and some chucks for the back wheels. Non-foulers for that second o2, and 4 grade-8 bolts, and 4 nuts. Like I said, you really only need two if you keep the studs in the cat, but I don't recommend that.

Installing the Megan Racing Header was a pretty straight forward and easy task… Everything came off within twenty minutes, and the downpipe drilling took about 10 minutes. Bolting it on, another 10. Took about 45 mins total install.

The support from Bill at KFX was great, and the Header was here in about 3 business days, even with the trouble of my address and crap. came out good with a little modification… So, to Bill, and KFX. thanks, a lot!

For everyone wondering, you need to purchase an o2 bung (18mm x 1.5) so you can install your second Oxygen Sensor downstream after your catalytic converter. It’s going to have to be welded onto your intermediate pipe (post cat), my recommendation is TIG welded, but MIG would be sufficient. I got my o2 bung from some website, and my buddy ended up just MIG welding it. The upstream o2 (air/fuel sensor) threads right into the new racing header via, a TIG welded bung on the right-hand side.

IMPORTANT!!! To lengthen the o2 sensor, you must splice in 16gauge wire, and it extend it as need be. It's going to be a little bit of a PITA routing it so it dosen't touch anything, but it's doable. I used the heat shield, and a zip tie up top to get rid of slack.

EVERYONE THAT DOSEN’T WANT AN ANNOYING CEL (check engine light), make sure you go with the non-fouler method, and purchases a pack of 2 18mm non-foulers. I’ve been CEL free since install.

The part number I utilized was HELP! #42009. Drill OUT ONLY ONE OF THE NON-FOULERS TO ½ INCH TO ACCOMADATE THE OXEGEN SESOR HEAD. I did this really quickly; I started the hole a few sizes smaller then ½inch and worked my way up to it. I put a 7/8th wrench on the floor of my garage, stood on it, and drilled it out. Took only about 5-10 minute, and is well worth it to get rid of the persistent CEL.

Once it’s drilled out attach the drilled non-fouler to the non-drilled non-fouler WITH ONE OF THE GASKETS!!! This way the air is minimal getting to the oxygen sensor. If done oppositely, the o2 sensor wont thread in. Make sure the small hole-ended one is the one that threads into your newly welded bung post-cat. Together they should look like this.

Now, installing the Header is just as easy as drilling out that non-fouler. Haha, there will be some drilling!!!
Megan supplies you with the needed gaskets, and everything pretty much ready to go. To be on the safe side, I opted to purchase new GRADE-8 bolts and nuts (4 of each) from my local Auto Parts Store, just in case they were a little bit of a pain to take off, turns out they WERE STUDS??? Replaced them aha. The Header is really nicely designed, TIG welded, equal-length, mandrel bent, 4-1 Header. Everything sanded and smoothed. Really good design, and seems to net some decent wheel horse.

I disconnected the down pipe from the cat first, then from the manifold.

Took the cover off the exhaust manifold, and took the manifold out thru the top of the car. Make sure you remove that bracket that used to hold the manifold to the engine.

The down pipe, there’s a little heat shield that you have to take off so you can push it out thru the back.

Bring the new manifold up thru the bottom. This takes two people. With that bracket off, it goes up thru easy.

We also took the cat off so we could match up the down pipe on that first, since it does take a little bit of work. You have to drill the holes out a little more, and I RECOMMEND HIGHLY, to take the studs out, and put real bolts in.

Studs out, bolts in, holes drilled a tad, bolts on like a dream!

It makes it MUCH easier. After getting the down pipe mounted to the cat, we bolted everything together, and put that heat shield back on underneath the down pipe. Pretty good.