The PCM will read the before and after cat readings and will throw the CEL for an emissions code as it will see too much N0X (oxides of nitrogen) in the after cat sensor, the computer will assume you either have a bad sensor or a faulty cat and will likely go into fail safe. Turn off the downstream oxygen sensors (B1/S2 and B2/S2) , the front ones are the only ones you need for A/F.