SES Code: P0140 Front or Rear O2?

Subject basicaly says it all. I have a 00' Grand Prix SE (3.1). I was told by 2 people (AutoZone and a friend that has a scanner) that this was the front O2 sensor. So I replaced the front O2. After about 30 minutes the light came back on. Had it scanned again and the same code came back. So I switched out the O2 under Wrty, reset the light again. And yet again after about 30 minutes the light came back on. After the second change and light reset, I'm starting to wonder if it's the rear. Any input?

Thanks

Randy

Odb-ii DTC error code P0140 is 02 Sensor CKT No Activity Bank 1, Sensor 2 which means it is a problem associated with the rear 02 sensor after the CAT convertor.