In my experience aftermarket cats generaly contain 50-60% less precious metals in them, to bring down the price. AKA they dont work as well as the ECU would like, and here in Colorado with IM240 testing they rarely pass the test. Did the light come on before or after you Seafoamed it? That alone could put enough contaminates in the exhaust to foul the o2 sensor. I would suggest a scope test of both o2 sensors, likely the front is failing, followed by using a "Top tier fuel" and no additives. http://www.toptiergas.com/retailers.html.