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so my 2003 honda civic lx didnt pass the smog because the previous owner tried to modify it
does anyone know a way to fix this for cheap
any after market cats that will pass
o2 sensor as well

 
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DIY Mechanical O2 simulator

Had a high flow cat on my 2002 ex that threw a P0420 code after some years of being installed . Mechanical O2 simulator did the trick to let me pass emissions. This trick also worked on my friend's 05 Honda Odyssey. As long as your state doesn't have a visual inspection, you're golden. If your cat is clogged, it's a bad idea, but it sounds like it's the opposite since previous owner tried to mod it (prolly hollow out the cat, if I had to guess). If you want an actual catalytic converter, however, I'm not 100% of any good aftermarket ones. I think magnaflow has a decent 49 or 50-state legal one? But I'm not sure how well it works or whether or not it'll throw a code after some time

That part number is still the same if you shop at autozone, but otherwise any 18mm spark plug non-fouler will work. Bonus points if you find the long ones instead of the pictured short ones, since no drilling is required if you do.

 
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i am running magnaflow, no isses