the car is a 98 accord lx with the f23a1, 124,xxx thousand miles. so yesterday on the way home my CEL came on. i brought it to my grandfathers shop and ran the code, it came up as a P0420 (Catalyst System Efficiency Below Threshold (Bank 1). i have another 98 accord that was hit in the front end so i took the catalytic converter off of that car and put it on mine. the car smokes more than usual now and smells like it has a test pipe on it. about an hour after putting the new cat on and clearing the CEL, it popped back up. i live on the top of a very large hill and now it seems like its "sputtering" and having a hard time making it up the hill, almost like it needs new plug wires. i have no idea why its acting like this. would a small exhaust leak cause it? i re-used the old cat gasket so my next step is to buy a new one of thoes. i really dont want to spend $100 bucks a piece for new 02 sensors and was wondering if i could use the defoler trick if that is indeed the problem. anyone have any ideas as to why my car is running like **** now?

 

I'd seafoam it every way imagineable.

 

hah well the thing is i just ran some 3M top end cleaner, fuel injector cleaner, and throttle body cleaner through it on friday and idk if thats y im in this mess now

 

Well how is the new cat on there? Where is the smoke coming from and what color is it? I would suggest just buying a new catalytic converter (even like an aftermarket) and put it in there. Verify the gaskets are good in the exhaust system and the leak is not coming from another area.

Seafoaming through the brake booster line may help some. As well as doing the regular maintenance on it.

 

well i just went outside and started my car and looked under it, i can feel the exhaust coming fom where the flex pipe connects with the cat. would that exhaust leak really make my car run like ****? i still dont understand why the heck it smokes so bad now, i mean it smokes a little n e way because i burn some oil but now its awful

 

If the exhaust is coming out right there, I'd go over the welds if you welded it, or the gaskets if you used them.

 

im going to order new gaskets. i bolted it on, it was from the same year accord so i didint need to weld it on

 

i dont know if my .02 will help any at all but i'm not really a fan of this seafoam or any other type of solvent like this for your engine. i highly doubt that the donut gaskets for the cat are whats causing your car to run bad. you may want to check your plugs, there is a possibility that the seafoaming could have caused you to foul a spark plug. also a clogged cat will cause your vehicle to run poorly as well.

 

Gaskets are the cheapest bit to replace. New catalytic converter obviously would be much more expensive, no reason to immediately go to it. O2 sensors could cause the CEL too, and still cheaper than a cat.

1) Seafoam (cheap)
2) Gaskets (cheap)
3) O2 sensors (cheap)
4) New cat (baller)


Refer to:
https://honda-tech.com/forums/honda-accord-1990-2002-2/p0420-catalyst-system-efficiency-below-threshold-bank1-o2-cat-1154743/

The recommendation there is to directly replace the cat. And seems the O2 sensors didn't help a few other users (FunnyVictor for example). I'm not much of a 6th gen'r and judging by the amount of posts in that thread I'd listen to them. Get a new cat (they're suggesting OEM for $300) and I'd make sure you get good gaskets to prevent a leak.