P0420 Catalyst System Efficiency Below Threshold

i got a new high flow cat about a month ago when i installed my turbo kit. you guys think the o2 sensor is bad? the high flow cat is a magnaflow and the o2 sensor is position after the cat. i've only had the high flow cat for a month

this seemed to happen after i installed my a/f guage i think. i redid the wiring, and about after a while of driving, the CEL will return. so i don't know what it could be. the a/f guage works fine. i have no idea what else it could be.

i read up that it would more likely be the cat than the o2 sensors. i found this simulator thing to trick the secondary o2 sensor. but wouldn't this throw off my a/f gauge too?

http://www.highrevmotorsports....=1108