High flow cat and engine light

I put a high flow cat on my 99 si (b16a2) and about 20 miles down the road my engine light came on. I talked to my local performance shop and they informed me that theres nothing i can do because of the high flow cat. Does anyone know where i can find something to fix that? i heard theres a resistor you can buy for the second o2 sensor thatll fix it. anyone know where i can get one or maybe another way to fix it? thanks.