After reading the threads about diagnosing a bad cat, I took my car for a hard run around the block tonight, then took a peek at the underside.

I stole the attached photo from https://www.rx8club.com/series-i-trouble-shooting-95/another-one-down-214269/#post3931099. Leaning down on the dirver's side and looking over, I see that same baseball-sized, glowing red ball or ring. It looks exactly like that, although I don't have the other glow towards the left shown in the photo. Just that ring.

Unlike the photo though, it was a very, very faint glow. I almost didn't see it, it took a second to even notice it.

So that's a 100% bad cat symptom? I'm at 78,000 miles, so if so I really need to push for a new one soon.

Thing is, I don't really have any terrible power loss symptoms. I occasionally get a slightly raspy sound and slightly less than usual power feeling when accelerating hard from 3,000 to 6,000, but not always. In fact, the run which made my cat glow was perfect, no loss of power, no noise.

Has anyone confirmed that healthy cats don't glow faintly?

 

that's not the only way to diagnose cat failure.
you may also want to remove the front side of the cat, pull down and look inside. If it looks damaged, time for a new cat! (Or mid-pipe )

 

Yup, I'm just not that mechanically inclined nor have a place to do it

I'm mostly wondering how insistent to be with Mazda when it goes in for other service. I have no CELs, so I fear they'll say it's fine. And hey, maybe it is?

They already diagnosed "arcing" coils (which will be replaced along with plugs), so I'm wondering if maybe the glow is from unburnt fuel because of the failing coils... and the cat is actually fine (but won't be for long unless I get the coils/plugs changed)?

 

Or let me ask it this way...

I realize not all bad cats show a glow, but are all glowing cats bad?

 

could be unburnt fuel. You could be running rich (these could be because of fouled plugs/ coils)
I'd say it's worth a shot to ask them about it, if you don't inspect it yourself.

edit: also, try cleaning your MAF and/or your air filter

 

Funny you'd say that, as I've been reading around the forum today, and just tonight cleaned the MAF per the advice threads I found... which is why I then took it on that hard run where I peeked at the cat when I got back.

I'm so proud of myself, lol! I know, you kids do all sorts of amazing things to your 8's. But cleaning the MAF was a big deal for me

The air filter I replaced a few months ago. Whomever designed that silly airbox (it's so hard to get the filter out!) is NOT my friend, lol.

 

If/when you get a CEL, watch for the "Catalyst failure" (I forget the Pxxx). I got mine replaced under warranty and i could not see any glowing. I also did not have any noticable power loss.

So, there's lots of varying symptoms but the one that makes the difference to mazda is the cel code.

 

For what it's worth to anyone, my catalytic tested fine for "efficiency" by Mazda, no codes or anything either... even though it glows (faintly) red after a hard drive at night.

So a glowing cat isn't a 100% sign it's bad apparently. I guess as others mentioned, it's probably the failing plugs/coils causing the glow.