there might be (1) exhaust pipe leaking; (2) O2 sensor bank 1 sensor 1 (located before cat); (3) O2 senser bank 1 sensor 2 (located after cat); and (4) the catalyst. use scope to measure the waveform of the two sensor and compare their forms to determine they are good or bad. if both are good and there is no leaking on exhaust pipe before the catalyst you need a new catalyst.