A catalttic inefficiency code will be set when the number of cross counts for the second o2s exceed a preset limit. See when exhaust first reaches the first o2s it has a varrying high and low oxygen content because the ecm is constantly sending a rich and lean command alternately. when the exhaust passes through the cat the oxygen is theoretically stripped form the NOX in the emissions and added to the co(poison) making co2(harmless) if the second o2s sends a rising and lowering voltage it sets the code for cat inefficiency. this could also be trigged by an exhaust leak but you probably need to replace the cat.