ok i could keep my self from telling you what i think about this problem but i think (im not sure) the code could be caused if the cat is not burning all or enough air/gaz mixture that did not burn during combustion phase cause the emission control system is quite exigent and will pop ur MIL on if there is too much mixture left after it went threw the Cat so if you cat is not giving the oem performance that the pcm have in memory u get a Mil light anyway i would just get an OEM Cat im pretty sure that is your problem however i know Oem cat cost alot maybe u want to look for a used one ... or leave the aftermarket cat and use O2 simulator its very cheap and well u just unplug secondary o2 and plug the simulator and try to put it as far from water and exhaust ... hide it somewere and make sur its gonna stay there
the simulator will provide the pcm the same mA signal as a well functioning cat