Most newer vehicles have two O2 sensors. One upstream (before the cat) and one downstream (after the cat). If the cat honey combs is broken apart inside, it could allow the exhaust to get past the cat faster and O2 sensor upstream will not be able to monitor properly or report insufficient gas/exhaust/02 mixture. If you got two cats then you got four O2 sensors as well. On mine over time after seeing 02 code and replacing the 02, I listened underneath the vehicle and I could hear the metal inside on one of the cat rattling and I have it replaced and it fixed the 02 code.