Cani Lupine - You said you put in NGK Ruthenium spark plugs and NGK coil packs. When did you do that? Was it just before these problems?
As to the running "rich." This is all speculation on my part. I have done my fair share of staring at a wideband gauge and carburetor tuning. But that doesn't make me an expert here. But my speculation is that those fuel trims you mention showing running sort of offset rich, that does tend to make sense with your ram air intake.
Because, if your setup is able to get some amount more air into the cylinders, the ECU is programmed to try to achieve stoich. If there is more air, it would have to add more fuel. Maybe this is the reason for the fuel trims to show some offset rich, to match the additional air with gas.
And ... this would lead to more power (more power baby). Perhaps this is why you feel like your car runs so well (after "learning").
I can't say I (re)read anything from your analysis that would tend to make me think your cat is failing. But I've been wrong before. The only experience I've had with cat failure was in an old car I had, when the cat began plugging the exhaust flow (what could cause it to fail like this plugging the exhaust flow could be a bunch of reasons I suppose). That car idled good and performed well at low rpm. But at any real demand it would fall on its face with no power. I think if you can take your car to 5,500 rpm and it pulls strong at WOT to 6,000+ rpm (max flow), I tend to doubt your cat is failing. Of course, I suppose there are other ways for a cat to fail. It could just not catalyze I suppose. But that wouldn't result in a lean / rich problem I don't believe. The lamda 0.893 reading (or some number like that) I tend to doubt will have much negative impact on the cat. As noted above, you're getting extra air from your ram air setup very likely. So in the end, your burn AFR should be good and in the range to not damage a cat (speculation on my part again).
I sort of mispoke about me saying I doubt your theories in one of my first replies. It wasn't accurate. It's not that I doubt your theories, all your posts seem to be very accurate to what I believe to be true. I just had my doubts that those theories were contributing to the code you were getting.
Last thing - After doing your exhaust leak repair, did you reset the computer again to put it through it's learning procedure again?
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.5 L/100 km ... 50.9 mpg (Imp)