2001 mustang GT stock exhaust with cats. Engine light was on when I bought it. P0420. My brother reset it. 47miles down the road, CEL light came on. My brother and his scanner is out of state so I went to autozone, once again P0420 code. Disconnected the battery numerous times. CEL light kept returning. First time at 47mile then 37, 25.. I bought a pair of mil eliminators from American muscle, because I need the car to pass inspection. Disconnected the battery for 25min while I installed them. Hooked the Battery back up and drove about 30miles and the light came on. Went to autozone for a code read, P0420... Any help would be appreciated.. This car is becoming a major headache. I don't have the money to bring it to a shop or to buy 4 new o2 sensors.