Actually a bad cat can indeed do this. Bad cat causes shift in measured O2, causing DME to shift mixture, and if after a while the O2 hasn't moved back to target, the check engine light comes on. E39 V8 cats more or less fail suddenly as whatever is in there holding the cat guts in place breaks. I have never heard of a cat failure that didn't rattle, so if you don't hear a rattle, its probably not the cat. Ask me how I know....
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'97 E39 540i six speed Aspen/Aubergine <-- Damn fast
'00 E39 M5 Carbon/LeMans <-- Faster
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