6th Gen Cat?

I just replaced my stock header with an aftermarket (98 DX, Hatch), and was surprised to find that the cat is mounted vertically, and is completely replaced by the header piping. I replaced the header regardless, but my intention was to work backward (replacing parts) to the muffler. Is the generally accepted idea that I add a cat in that line, or was I just out of line getting a 4-2-1 header? If it is the general rule to add a cat in that line, are 6th gen hatchback cat-back systems shortened accordingly?

I am confused..it might just be the carbon monoxide.

I spoke with some technicians at Magnaflow - this is what they had to say:

"Usually you wouldn't use a header in that application. If you move the cat you will throw a check engine light so you will need to run 02 simulators. Thanks for your interest in Magnaflow."

How does moving the cat throw a check engine light? Is there an oxygen sensor SENSOR, that monitors the location of the oxygen sensor? How does the computer know that the cat has moved?

Any help would be greatly appreciated..