You need MIL eliminators. Some people get lucky, and codes wont pop up for weeks, some people don't, and codes pop up every 5 minutes. You can grab them at AM, only need 1 set and they plug into your rear o2 sensors on your mid-pipe.
Out of business
1998 Mustang GT
Atlantic Blue (5 Speed)
Stacking Racing Plenum, Polished Aluminium CAI, MAC O/R H Pipe, Flowmaster American Thunder Catback Exhaust System, upgraded lash adjusters