2001 Accord EX, 5 speed, 4 cyl.

Had the P0420 code (catalyst system efficiency below threshold bank 1).

After investigation, found that the flex pipe bellows had cracked and leaking exhaust (see pic, hard to see but where the cracks are there where little cracks underneath).

Replaced flex pipe (section from manifold to cat), new bolts (also had to replace the "out" side of the cat's bolts <see pic> two nuts had literally disintegrated and there was a huge gap) , new gaskets. Cleared code. Tested and NO leaks present from the manifold back. Drove it for about 50 miles and CEL back again.

Same P0420 code.

So, where do I go from there?

Could be O2 sensor? checked it when working this problem, looked ok, not really sure what to look for, the tip was all white, assuming that was normal.

Could be the cat itself? When I had it out, obviously all kinds of white stuff came out, I've heard the test about rattling to see if it's bad. It did not rattle but there was some stuff moving around inside - I assume more emissions but is that a sign of a bad cat?

 

The exhaust leak can throw off the 2nd O2 sensor's measurement, but seems like it would be OK when fixed. I don't know if operating like that would have damaged the cat?

Did you clear the code with a scan tool or did you pull fuses or battery cables? If you used a scan tool, you might want to completely reset your ECU by pulling fuse #13 in the fusebox near the right-front door hinges. Maybe it has to be forced to re-learn some operating parameters.

 

I just disconnected the battery to reset the codes.

I am leaning towards a new O2 sensor if it doesn't clear. 240K miles on the original isn't a bad thing, it's probably just tired by now.

I'll try the fuse and see if that keeps it off.

 

Pulling that fuse does EXACTLY the same thing as disconnecting the battery. Only you don't have to remember your stereo anti-theft code.

So a tired old O2 sensor makes a lot of sense.

 

Fortunately I have my radio code memorized lol

Pulled it, had to run the kids somewhere, ran a little more, so far so good. Will be able to tell tomorrow, have a meeting and should put about 60 miles round trip on it. If it comes back on, O2 sensor it will be.

 

Update:

After 60 miles the CEL came back on, no luck.

However, since I drove it longer today I did hear a rattle under the car. It sounded just like when my heat shield went a couple years ago. But would only hear it when hitting the brakes in traffic, by the time I got home it was a steady noise. I put the car up on ramps and it was still rattling pretty good until the car was off.

The heat shield looks fine (poked it with a stick lol) and it looks nice and tight.

I am guessing the cat might have had something broken loose inside when I was banging bolts in?

I have a new O2 sensor to put in, but I think I'll pop the cat off to see if that is the rattle, if it is, obviously a new one is in order?

 

Broken ceramic monolith inside the catalytic converter means you need a new one. But sometimes an exhaust pipe that's beginning to crack & leak can make all sorts of sounds too. Make sure you check to rule that out.

Exhaust leaking out & blowing onto the outside of the O2 sensor will throw off the measurement. (Sensor actually measures the oxygen DIFFERENCE outside vs. inside of pipe.)