Signs you need a new catalytic converter when you get a P0420 code (with pics)
 

I have been posting my P0420 saga (catalytic system efficiency below threshold bank 1) that everyone seems to get at some point.

It is a 2001 Accord EX, 5 speed, 5 cyl, ULEV, 240K. Obviously trying to spend the least amount of money possible this was one of the few times I actually did a step by step process of elimination instead of just replacing parts, I thought I would put it here to inspire others, lol

The P0420 can be many things depending on who you talk to or read about. My assumption was it obviously had something to do with the catalytic converter/O2 sensor.

First, checked for any exhaust leaks. There was a slight smell of exhaust but not overwhelming but with a car with 240K miles I didn't really think much of it. Ran some SeaFoam through the PCV vacuum. Let it sit, started it up (she'll smoke really well and a piece of cake to find leaks). The flex pipe looked like an industrial smoke stack (see pic, you can see the cracks in the bellows and underneath the mesh like material), hmmm I think there is a leak I said to myself. So I started there. While repairing the flex pipe, I also found that the nuts on the muffler side of the converter had disintegrated, literally and was only being held on by one nut (see pic) causing a huge gap. Those were fixed too (fortunately I had ordered extra bolts just in case).

Walker flex pipe $72 (Rock Auto)
Gaskets/Nuts/bolts $42 (Majestic Honda)

Leak fixed, code reset, no more exhaust smell. CEL came back on.

Figured I'd reset the code again, just to see if it jogs anything in the computer. Came back on after about 60 miles.

Was figuring on the O2 sensor. When I took it out to put the new flex pipe in it looked like it should but it's been in there 12 years, he was probably pretty tired so I decided to replace it. Stopped at Napa on the way home.

Denso O2 sensor $42

Meanwhile.............

I start to hear a rattle. If you've ever had a loose heat shield, it sounded like that. Only when braking. Figured it was the heat shield. By the time I got home it was rattling pretty good and when I put the car on the ramps it was still rattling. The heat shield was fine.

Tapping the cat with my hand (after it cooled off of course, I took a nap, it was nice), could hear the rattle, rolling my eyes I was pretty sure it was the cat. Took it all apart and sure enough the insides were cracked into three pieces and rattling around (see pic). How it happened? I'm not sure because when I put the cat back on after fixing the flex pipe it didn't rattle at all. Maybe when I was tapping bolts into it it all got knocked loose.

So, after process of elimination.

A new catalytic converter is ordered.

Magnaflow converter $135 (Rock Auto)
Gaskets $4 (Rock Auto)

Now there is a silver lining. Was able to sell the old cat for recycling for $130.

Flex pipe/gaskets $114
O2 sensor $42
Cat/gaskets $139

less recycled cat -$130

Total cost $165.

Thank you hondaaccordforum!!! Without your help I might be at the mechanic and $1000 into it.

Those are some great pic's......yes I know you hate it because of the $'s involved to repair.....but others should learn from this.

Thank you for posting what worked :)

I missed this thread when originally posted. A couple of questions:

How has Walker converter held up? Any further problems.

Funny how your damaged converter was worth $130 and you were able to buy a replacement for same cost. Not sure how Walker able to deliver cat for such a low price w/ a used salvage value is equal!

good luck

The flex pipe was Walker, the cat was Magnaflow which I got from RockAuto, which you probably know, has pretty cheap prices.

All the aftermarket stuff has been fine, no issues at all, she's purring as good as ever.

We buy scrap catalytic converters Nation Wide! Prices Updated Monthly on our Catalog. is the company, they're actually only about an hour from me in PA. Text them a picture, they send a quote, request a shipping label (they pay) and send it back, get paid by check or PayPal in 2 days, a no brainer. Sold one off my 1993 Volvo for $90. Obviously aftermarket stuff won't get much. They specialize in recycling cats, not sure why it was $130 but I wasn't complaining lol.

I have one a buddy pulled off a Hyundai I'm texting them tomorrow.