Well my rattling catalytic converter finally got to me too much. So I got me a new Magnaflow direct fit cat and some inexpensive Summit Racing house brand cat-back exhaust. Other then getting the O2 sensors out it was pretty easy and somewhat painless. The test drive went great...felt like it had a tiny bit more power and I am happy with it all in all. But...after helping my buddy clean up his shop and having a celebratory bee, on my way home my CEL (check engine light) came on. My rig is a 97 TJ 4.0L, I did the key trick to get what codes are thrown and this is what came up: 12, 21, 55. Easy enough, 12 is a disconnected battery since the last 50 key cycles and 55 is the end of the test...but 21 is an O2 sensor. My Jeep has never thrown a O2 sensor code so now what? My Jeep is driving fine, running fine. I replaced the exhaust last Wednesday and today is Saturday. Light is still on, I checked the connections on both my O2 sensors and they are good, no wire damage either. Last night I disconnected my negative post on my battery and it sat for about 6 hours before I hooked it back up. The light was out for about 2 mins then it came back on before I got out on the main road. Anyone have any ideas? Should I just change the sensors or wait to see if it goes out on it's own?