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When selecting an aftermarket Downpipe, MHD disables the health check of the cat. That's all it does or should be doing. It does not prevent other CELs to be displayed, this would be very wrong, as you point out as well.
The reason you find some historical codes stored is that not every fault is a CEL. There are thousands of possible fault codes, only a subset however are classified as CEL- triggers.
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