Well from what I recall reading, I may be incorrect, but since you want the cat hot when you go in to take the test having the IC cool the air makes it a bit harder to pass. I don't think it was supposed to make it fail maiserably or anything but the hotter, as opposed to cooler, exhaust gases...
Wasn't it supposed to be harder to pass because the bigger IC cools the air flow and makes it harder to burn all he exhaust gases? I forget where I heard that though.
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