Why don't you unbolt the cat from the header pipe and see if it helps? It's two bolts.... And yes a totally clogged cat can cause no start/low power.
depending on where you live and if an o2 sensor plugs into it, hollow it out and go, mine is hollowed out and i have it modified so i don't get a CEL, i know the tree huggers hate me but i couldn't find a high flow cat and a new one is really expensive.
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I Need some WRX, 350Z, 240SX, Really any car owner to let me do R&D for Ground Kits, Please Let me See the layouts!!!