My guess is to go to autozone and get the codes read. Then post the codes here and we could tell u what they are and possible causes and yes maybe ur exhaust mod affected the o2 or set off an o2 code nothing to worry about when u reset the ecu. Atleast thats what i would do is reset the ecu to relearn with the bew exhaust
1996 Mustang GT, 17x9 front and 17x10.5 cobra r replica rims, 255/40/17 front 285/40/17 rear at the moment,
o/r x-pipe flow master 40 series dump downs, cai/ram intake, 75mm c&l throttle body and plenum, pi heads and intake swap, msd wires, stock gears, untuned at the moment, no dyno numbers for now.