I don't know of any hiflow pipes that have four cats but I guess there could be - they usually have two(like yours has). Your 96GT in stock form has 6 cats total. One of the cats could be partially be blocked which would still allow for some exhaust flow. How long has the check engine light been on? Did they check the O2 sensors? It would make sense that the light would come on if something was wrong with the cat(s) but you wouldn't need MILs unless you were running an offroad pipe with no cats at all. You could try resetting the computer by disconnecting the battery but if it's throwing codes the light will most likely stay on.
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!