So I've had the Perrin divorced downpipe with cats on for around four months now. All was well OBD readiness was set on all monitors, and no CEL. I just recently installed the Perrin uppipe with EGT sensor delete, and even installed the 2.2k ohm resistor in place of the sensor to fool the brain... Last night I threw a P0420 catalyst inefficiency code. I know it's not the cats because I HAVE cats and had no CEL for four months. Is it coincidentally a bad downstream sensor, or could something else be the fault?