99 civic ex D16Y8
Last fall, I had this same code and had the cat replaced with what im guessing is a cheapo, (my mechanic said he ordered it from Reno) it costs $250 installed. When I bought the car, it had an aftermarket cat welded on so I know it's been replaced before.

I replaced dizzy last fall also with a new Richporter.

The car drives great, just seems a bit underpowered at low RPMs, until about 3000. But it never stalls out and it doesn't shake at idle. The only real symptom is that it feels underpowered.

CEL came on last night, code was PO420.
When the cat was replaced, the O2 sensors were not replaced. I think I'm gonna pull the exhaust manifold and replace the gasket, but I don't think that is the problem. What do you guys think?

If it turns out the cat is bad, what are some possible reason why cats are continuously failing in my car?

 

Well, I have to drive to the bay area today for work and I'm a little nervous. My cat isn't totally clogged because theres still quite a bit of exhaust coming out the tail pipe.

Things I'm pretty sure of:
It can't be coolant because I just overhauled my cooling system and changed hg and had head decked last year. And my coolant level has been perfect ever since so theres no leaks, no way any is getting into the cat.
Dizzy has less then 10k miles on it, spark plugs have like 20k but they still look good, wires are good too.
There is an oil leak in the main seal, it's not terrible, only a couple drops a day. Idk if theres anyway oil could be getting into the cylinders because of this but I doubt it.
I hear a very faint hissing or whistling noise coming from under the car, but it is louder as you move towards the muffler and away from the cat. Sounds like a tiny exhaust leak to me, but it's post-cat so it shouldn't have thrown the CEL.

I have to go the bay on friday as well, but I think this weekend, I'm going to cut the cat out and straight pipe it until I have to smog. I don't want to put another fail cat in, but the OEM is $1200, and I just can't afford that right now. My concern here is that there is a heated O2 sensor that sits in the cat, if I remove the cat and let the O2 sensor just hang there, will the computer **** with my air/fuel ratio? Could this be dangerous? Or should I just put the O2 sensor inside the straight pipe so it can atleast get a reading?

 

Just got back from San Leandro. Bay area traffic is just awful during rush hour, there was about a 20 minute period where all i did was brake and take my foot of the brake to idle. Near the end of this 20 minute period, I noticed my temp gauge go up above normal operating temp to about 70% on the gauge. Thank god, traffic started to move after that, and the temp went back down to normal and stayed there for the rest of the drive.

First thing I did when I got home was leave the car running to see if the fan was working. It did come on. I check coolant overflow tank and it was about half full. Right now I'm waiting for the car to cool down so I can open the radiator and see if it's actually full and to look for signs of a HG breach (In which case im screwed). Will repost what I find.

 

Radiator looks good, it was full to the top and there was no sludge.

Did a further inspection of the engine compartment and noticed there is a layer of soot over EVERYTHING. it looks like exhaust. So now Im thinking exhaust leak, maybe the header gasket is breached.

That would explain why my car got hot. Explains all the soot everywhere. Would explain the general lack of power I feel, even before the PO420 code. Idk if this is true, but I heard it could cause the cat to fail after a long period of time.

Come on forum masters, please tell me your thoughts on this.
What a good way to check for exhaust leak? Some kind of vacuum test? Do they still sell skunk oil or something similar?

 

look at the coolant first.. it probably has air pocket in it, do this first.

If your smog is not due anytime soon, install a non-fouler on the O2 and that may take care of the code.

Also, which bank is indicated on the code??

 

Check for exhaust leak by temporarily blocking the tailpipe with the engine idling. Small leaks will make a hissing sound as the pressure builds up. Big leaks will prevent any pressure from building up at all.

 

Theres no air in the coolant system, Ive been driving it like that for almost year with no problems until now, plus I just inspected radiator and overflow.

Smogs not due till May '14. Thanks for the non-fouler idea, i'll look more into that.

Code was on Bank 1

 

Doing this first thing in the morning, as it is dark here and I have no garage to work in.

 

The second sensor (after the cat) is only there to check the performance of the cat and throw the P0420 code if it doesn't seem to be working. Only the first sensor near the engine controls the air-fuel ratio. Running with a bad first sensor makes the engine run rich, which is bad for the cat.

Both sensors are in "bank 1". Codes for "bank 2" will never appear on a Civic. They are for V6 and V8 cars which have sensors on both manifolds and possibly separate cats for each side.

 

Well I replaced the spark plugs today because I know theyre not perfect and they were only like $9. The old plugs did have minor black scorching at end of the threading (the ring between the contact points and the threading), I think that is some indication of running rich, but inconclusive.

Had my buddy plug the tail pipe for 2 minutes straight and honestly, I couldn't even tell it was plugged, the engine did not die down in the slightest, so now I'm 100% sure theres an exhaust leak. Waiting for car to cool again, then I'm gonna remove heat shield, jack it up and run my hands along the entire exhaust system.