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I have a 2003 Civic LX. The LX model has a exhaust manifold/catalytic converter combination unit, unlike other models which have the two separated. I would really like to get more power out of my ride. Aside from a CAI, I know I can only go so far without converting to a wider exhaust system for better airflow. I have found no such aftermarket manifold/cat part with wider pipes. If I convert to the EX style separate exhaust system in order to throw in a catback exhaust with a wider exhaust manifold pipe setup, will the car then throw a P0420 code and fail emissions?

 
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As long as you re-route your down stream O2 harness to connect with the sensor, your computer would not care.

On a header equipped EX, they put that sensor harness out the bottom of the firewall, I would think you could simply split open your harness wire loom and pull that sensor free. Maybe test on a junkyard harness.

 
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The new exhaust will still have a cat correct?

 
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https://www.civicforums.com/forums/2...t-diy-faq.html

 
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As far as changing the wiring harnesses to have the proper length, I should just be able to use one from any of the other sub models, correct?

Yes it would. It would be moved to the underside of the car like the other models. I would most likely use a high-flow cat to compliment the wider pipes. That way there's less restriction. I was thinking of going with a MagnaFlow. Only thing is, in order to go up 2.5 inches, it has to be a universal cat. Hoping that wouldn't be a problem. I'm trying to do this so when it's all finished, it won't set off the CEL

 
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So as long as you have a cat and move the sensor you'll be ok. You can also buy o2 sensor extension harnesses. The universal cat may only last a year before the computer deems it "bad" Honda computers are really picky. You also won't see any power improvement, but it will be shiny and sound different

 
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Would I just be better off with a direct fit in that case to avoid welding? And use that with the aftermarket exhaust manifold? Definitely don't want to do the extra work of welding on a high flow if it won't increase performance. I'm not too worried about sound. The catback will probably change the sound anyways

 
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I guess it would depend on what's cheaper and what your welding skill is like. Although an expensive aftermarket cat would last longer than a cheap universal that you would have to weld flanges on

 
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Do you know a good place to find decently priced aftermarket cats? Doesn't need to be anything amazing, since it won't help performance much, as long as it pairs with the 2.5 inch headers. I've been looking, but I keep coming up with mostly test pipes or cats that need flanges welded. My welding is par to be honest. I suppose it would be better just get something that doesn't require it.

 
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Rockauto has good prices and cheap shipping. I'm surprised none of the exhaust kits come with a cat

 
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Looked on RockAuto and before I decided to buy one, I made one more search and found this little setup. Any thoughts? Appreciate all the input by the way. Never done this with a Civic before.
https://www.ebay.com/itm/For-01-05-H...4383.l4275.c10

Gonna be using a Megan header as well. Heard good things for being cheaply priced.
https://shop.redline360.com/products...YaAga-EALw_wcB

 
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Never heard of that brand, looks like a decent kit though

 
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Too bad random technology isn't a thing anymore, They made a great high flow downpipe for the D17

OP, if you get something that still throws a P0420 CEL, you can always do the mechanical O2 simulator trick. My 2002 passed smog with that O2 sim after my engine decided to kill my cat.

edit: here's the quick and dirty way to make a mechanical O2 simulator

1. Buy a spark plug non fouler or two. Two if they're the short kind. one if it's the long kind.
2. If you bought the two short ones, take one of them and drill it out with a 1/2" drill bit, then thread that drilled out one into the non-drilled one.
3. Thread that non-fouler assembly into the downstream O2 sensor port.
4. Thread O2 sensor into non-fouler assembly.
5. Plug in electrically and clear codes. Drive around and keep an eye on CEL.

edit: here's the link to the DIY guide on the forum. After I went shopping for one, I found that there's a long spark plug non-fouler that negates the need to drill, and still fits the entire sensor part of the O2 sensor.
DIY Mechanical O2 Simulator

 
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I'll be giving that a shot then.

Thanks for that tip by the way! I've heard of the bypass trick, but I never knew how it was done.

 
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new edit on my post. Added link to the actual DIY guide we have