SIvic2000

driving down the highway, my check engine light came on. ran a diagnostic and error code p0420 showed up. which is catalyist deficiency below threshold. i know the cat is good, i just put it on my car about 4 months ago, brand new. along with a whole apexi exhaust. any suggestions what it could be? i drive a 2000 civic si, b16, v tec.