i know you said you repace it but it sounds like the IACv. did you check the wiring ? if you have the proper apecs you should be able to see if its in yoru wiring or ecu. vaccum leak(major)?

also its trhowing a o2 code
its been over a year since i even tried to mess with emmision code but
i think its the o2 after the cat thats might be throwinf the p0420
doesnt mean you have a BAD CAT but you casnt rule outt hat possebility