GM PCM's can be very finicky with the O2 sensors. Particularly on these engines where the PCM is in charge of sooo many parameters with very little tolerance for error. It relys on certain parameters of feedback for both normal monitoring of O2 readings, as well as running I/M system tests (I.E. your readiness monitors).
There is NOTHING wrong with Denso or Bosch O2 sensors (I've used both before), but time and time again, someone goes to AutoZone (or other parts store), saves themselves $15 - $20 by buying one of these non-GM sensors, fixing their original issue, only to end up with new issues due to this non-oem sensor. Finally they bring it to us (the dealer), where we obtain the codes it's throwing/thrown (from the history files), and by reviewing any freeze frame data available, many times it zero's in something fishy with an O2 sensor. So we get the car up in the air, and sure enough, a Bosch or Denso, or other brand HO2S is in there. Swap it out with an AC Delco unit, clear the code(s), clear the history file (If applicable), and the problem is solved.
Moral of story, this is one of those situations where the OEM part is truly the best part, designed by the people who designed the car, for the PCM, Engine, and application it's in.
I totally agree! My hobby car is a 95 Impala SS, with a stroker motor. On my Impala forum, literally hundreds of people have had problems using non OEM O2 sensors. It was and is, hit and miss, on whether your PCM would like or dislike other brands.