2000 Civic SI check engine light

I have a 2000 Honda Civic SI and I recently had a muffler shop install a cat-back exhaust system on it. A month later, my check engine light came on. I had the OBDII read and it said "catalyst efficiency below threshold." So i had the muffler shop put a new catalytic converter on, which made the check engine light go off for about another month. But it came on again!!! Why is this happening??? Should I purchase a specific type of catalytic converter??? Are other people with SI's experiencing this problem???

it could be the sensor, the calibration is out of whack. if htey are cheap i would try and buy a new one. is yoru car running rich at all ? what kind of cat did they put on it, how generic was it ?

I am not sure if it is running rich. How would I know (I am an idiot about certain things.) I think the cat. was pretty generic. It came from a place called muffler brothers (out of dayton Ohio.) Is there any way for me to tell if the calibration is out of whack or will there be a label/barcode on the cat itself so i can research it?

theres something going on with your cat, because i have an 00 civic ex, and when i removed the cat and put a test pipe on, my check engine light would come on after about a month. and the ecu is really slow catching stuff like that. but if it still does not help. you can buy an O2 simulator which makes ur ecu think you have a catalytic converter and everything is running ok. thats what i have on mine with no catalytic convertor and i pass inspection without a hassel. Good Luck Bro.