96dxhatch

Hi.
My first post and newb to forum/civics.
Im helping my nephew get our 96 civic back on the road.
It has 104k miles and automatic.
Runs perfect but...
The other day it started running bad and stalled.
Took it to shop and codes p0304 p0135 p0420.
I guess a spark plug wire popped off and it started running rough.
Once back on it runs normal again except a little rough at idle.
I drove it 60 miles yesterday. highway, city, and mountains.
Ran perfect. Temps stayed normal.
So my question is.
Would it run so good if cat is bad?
Im in SF bay area, California and noticed cats are pricey and with manifold.
Is there a reason to buy both?
Im trying to spend as little $ as possible.
If I do end up buying new catalytic converter is there a higher flowing option and
still CA legal which doesn't sound terrible?
I need to fix soon as well because Its time to register and smog it!
Thanks for listening!!

Colin42

The car will run fine with a bad cat, if the cat is clogged then it will have a problem. Cat and manifold is one integrated unit, could try buy and sell pages or junkyards for a used one but hard to prove if the used cat is any good.

ezone

The cat code means it's lost efficiency (Oxygen storage capacity).
Engine power may only be affected if the cat breaks apart or otherwise becomes a severe restriction in the exhaust.

You will need a California CARB certified compliant cat in order to pass the California emissions check. That eliminates all the cheapest cat choices.

You will need to fix the O2 sensor code (P0135) as well, and you better fix that before you replace the cat. The cat should be the last thing replaced, make sure the engine runs correctly before you do.

Make certain no more plug wires can "pop off" again, because that can ruin even a brand new cat in only a matter of minutes.