The cat will clean up excess HC emissions - for awhile. Once it plugs up, you'll have to replace it too.
The O2 sensor can measure incorrectly, without failing to function altogether. If the ECU THINKS it's ok, it won't trigger a code. Of course, if the ECU isn't functioning, there won't be a code either. The non-loop control of the CIS injection will still allow fairly normal apparent operation.