LONG story short, about 2 years ago I replaced the pre cat O2 sensor with a denso. About a month ago I replaced the cat and the rear O2 sensor. I also cleaned out the EGR system and ran a can of Chemtool thru the engine. I've driven almost 1000 miles and today the P0420 code reared it's ugly head again. I can't find any air leaks and the air filter isn't the issue. I'm stumped as to what to do. I did not use a Honda Cat, but did use Denso O2 sensors. The car runs fine, just has the CEL on. I've used the search function here, but after checking what has been suggested here I am, still dealing with this dadblasted CEL. If there is something I've not checked I'm all ears.

Thanks for the help.

 

Do you still have the old catalytic converter?

 

You might investigate for a cracked exhaust manifold. This could allow air to enter and cause P0420. Remove heat shield and inspect exhaust manifold while running some engine cleaner through the engine to generate lots of smoke.

good luck

 

No, it was the original one that was held together by JB Weld and rust. Lol. It fell into 3 pieces when it was cut off. Regardless of this code it needed to be replaced after 222,000 miles.

 

Ok. The exhaust manifold is the pipe that comes down from the engine and connects to the cat right? Thx for the help everyone.

 

Just for the sake of terminology, the manifold is the first part that bolts onto the engine. You'll see a bolted flange down low between the engine & radiator. The "A-pipe" is the next pipe that goes between the manifold & the cat-converter.

 

OK, so I finally got around to looking at this in depth. I tried to make the engine smoke but I must be an idiot because despite spraying throttle body cleaner into the intake it just would smoke. It would kind of bog down a little bit but never did die or smoke. What do I have to do? I would spray cleaner on each of the exhaust connections from just past the first O2 sensor to the muffler and couldn't get any smoke. I did not notice any obvious cracks or spots where air might be getting in/out.

Then, as instructed above I took off the manifold cover. Just above the pre-cat O2 sensor I found what looks like a crack. (see picture below) I sprayed cleaner on it to see if it would suck it in or show an obvious sign of air moving in or out with no luck. I then saturated it with cleaner to see if any would show out the exhaust with no luck. IF it is a crack, it is a hairline crack and I have no idea if this is the cause of my CEL or not.

Is there any other way to test this crack to see if it is the problem? I wouldn't be surprised that it is because A) this is the original exhaust manifold and B) it took a long time for the code to be thrown after my cat/rear O2 sensor replacement. If memory serves it takes 3 cycles of 45 minutes of incorrect emissions to throw this code correct? I have a 50 mile commute to work each way and it took over a month for it throw the P0420.

So thats the current status, thank you for any help you can provide in nailing down the issue.

 

I've found exhaust leaks by holding a rag over the tailpipe. Have someone wad up a rag & hold it over the tailpipe. Don't seal it up so much that it stalls the engine, but that makes a lot more back-pressure in the exhaust pipe. Then all the leaks start whistling.

 

If it's a crack, it may get larger as manifold heats. Use seafoam or other cleaner to generate lots of smoke.

A can of spray dye penetrant will allow the crack to be seen.

good luck

 

I took off the vacumm line thats attached to the PCV valve but over on the top of the intake (not the pcv valve hole) and sprayed throttle body cleaner in there. Is there a better place to spray it in for more smoke? I'll pick up some sea foam tomorrow and see if I can get it to work. I guess I could take the exhaust manifold off and see if the crack is present on the inside?

I did try the rag over the tailpipe but was unable to notice anything different.

I hate chasing problems like this! Thank you for the continued help guys.