p0420 help please

So I've had this g20t 2001 for six monthes now and when I first got it was the alternator was aftermarket got an oem fixed that prob passed smog here in ca because its required. About a monthe after smog the p0420 came on thought it was a egr tube off the manifold so i got an oem from dealer didnt remove swapped the o2 sensor off the manifold/precat went away for 4monthes. Last week i tried a spectre drop in filter bad idea it kicked on the p0420 back on cleaned the maf since its happened to my b14 and it worked for 200 now its back on no rough idle no sputtering just bad mileage and cel light on any input would help