I have a 1997 960 that the check engine light keeps coming on for the same reason, indicating that the catalytic convertor is not operating properly. I have replaced the cat twice, both oxygen sensors, the intake manifold, the ecu, checked the wiring to the oxygen sensors. None of this is working. My tech (a Volvo specialist) has determined that the rear oxygen sensor, which should be more flatlined, is switching at exactly the same amplitude as the front oxygen sensor. When the car is driven under hard acceleration, the rear O2 sensor reads the way it is supposed to. Any ideas on why?