new2mustangs

I recently purchased a 2002 mustang gt it has had the catalytic converters removed and replaced with magnaflow exhaust. I did not know this until after i bought it and the service engine soon light came on. I had the computer read and it showed bank 1 sensor 2 o2 sensor was bad. I bought some mil elliminators but the light came back on after about 15 mi of driving. My question is do I need 4 mil elliminators or is ther something else that needs to be done. I placed the 2 that I bought on the rear to sensors then moved one of them to the front sensor when the light came on the first time but it did not help. Any suggestins would be appreciated.