So what does a clogged catalytic converter look like?

The internet as a whole is not really giving me any info here.  Have you seen a clogged cat?  What's it look like?  What am I looking for here?  Is a clogged cat super obviously blocked and I'll know it when I see it?

This is in regards to my RX8.  They're well known for clogging cats, but I don't know if I am looking for total blockage or what.

Here is a brand new version of the cat in question, but NOT the exact one on the car.  This one is headed to my Miata but made for an easy comparison photo:

Unfortunately I didn't have the foresight to grab a 'before' photo of the one in the RX8.  It currently looks like the pics below, after 6,000 miles, 120 autocross runs and (10) 20-minute track sessions.  I wouldn't mind replacing that often it if I had to but I don't know what to base that decision on.

Inlet

Inlet up close

Inlet up close again

Outlet:

My gut feeling is that this is 'fine' but at the same time there's definitely some buildup on the inlet edge, the outlet is notably cleaner.  Not sure if this is a case where ANY visible buildup means the tubes are likely clogged in the center...or if it's acceptable to just keep an eye on the inlet face and observe if it gets worse over time.  Thoughts?

I realized after putting the pipe back on, that I should have rammed a bright light in one end and observed from the other.  Might have time to do that tonight but still curious what people consider an acceptable amount of visible buildup or blockage.

Looks good to me, but yes, a light from the other side would help.  They're hard to see through unless you get the exact angle which means it takes a while to scan through every passage.  It's also possible (but unlikely) you have a fractured section in the middle.  Usually that is evident when you shake it, but it is possible that a chunk broke off inside and wedged crooked.

I had one that was clogged because the core had broken and was now sitting sideways. Diagnosis was easy because parts fell out. I used to have a piece of it sitting on my desk.

If it's a straight shot through the cat, shining a light through it does work. If it's clogged badly enough for you to make you think it was clogged, it should be pretty easy to spot.

I also had a metallic core cat come loose from the housing and start bouncing around. Every time it hit the wall of the housing, it compressed a bit. Ended up as a slug of solid metal sitting inside the housing. Very free flowing overall, not very effective as a cat.

Here was the one in my miata, as Keith said it was detached and bouncing around.

A clogged cat usually looks something like a miniature bee's nest in which some of the cells have been filled and capped off by the bees.

In reply to johndej :

Hey, that happened to my Miata, too. The quick fix involved a chisel and a hammer. (But, yes, a fresh cat was installed very soon after.)

I had a high-flow, metal core Miata cat start to come apart and blow chunks out the back, after much track abuse. :)

It didn't impact flow, although it presumably did impact the ability to do what it was supposed to.

You need to look at them with a pressure gauge, not an eyeball.  It's usually not the leading edges that plug up, but the middle, where all the heat is.

Unless the brick is broken up and sideways in the shell, but you can diagnose those with a hammer on the shell.

I had two cats fail on my RX-8. I replaced the second one myself. It didn't look clogged. Replacing it resolved the P0420 code. The only other symptom was a bit of additional higher rpm hesitation. There are four things that move in the intake tract as you rev out a rx8. You can feel some of those transitions. It just felt like a slightly more abrupt version of one of those changes.

I had a cat fail on a 6th gen Camaro too. P0420 code.

My vote is a cat isn't clogged unless you have a P0420.

Note that a P0420 isn't measuring airflow, it's measuring effectiveness. That's why you can fail with a cat that doesn't look like it has a problem, or how you can have a clogged cat that still passes the test.

Thanks for the input, seems like the answer is I need to evaluate with either with airflow testing or with a light source looking through it to have a solid answer.

It's a long story but this particular car will never have a P0420 code anyway.  As far as RX8-specific stuff, I do feel the 'normal' torque dips/transitions at high rpm, mainly in 4th gear, which is part of what prompted a checkup on the cat.  I can swap on a straight pipe to see if the dips change in magnitude but have to wait til the next track day to really test that.  They're harder to feel in 3rd and winding out 3rd is still really cookin' on the street and I'd rather not.  Top of 2nd gear I can barely feel the transitions at all, nevermind a difference in two runs after swapping pipes.  The RPM's climb too fast for the butt dyno or data to notice the dips.

I had the ignition rotor come loose on an '89 Civic and it dumped a lot of fuel into the cat as I limped it off the road. I suspect that there was fire inside as the center console was hot and a lot of the undercoating had burned away. The inside of the cat melted down to a big blob of metal.

I ran a hole saw down the middle of it and used it for another five years or so.

Raw fuel is a great way to kill a cat. We often see cat failures follow a coil failure in fairly short order.

In reply to Keith Tanner :

I had a six cylinder S80 set its carpeting on fire due to a bad coil.

I have no idea why Volvo didn't use the "kill injector when misfire counts exceed X" feature in Motronic.

Fwiw, you have a metallic cat, which fails very differently than ceramic ones do. And they tolerate much higher temps before the substrate fails. 

I once had a confusing intermittent power loss in a car, which ended up being the cat guts.  It came loose, then bounced around until it became about baseball size and shape.  Hit the throttle hard, it would roll up against the outlet, with the passages running sideways, and block flow.  Back off the throttle, shift to neutral, key off for a second then back on, and it was good again, until you hit the throttle hard.