Engine light on after o2 sensor replacement

Hi,

I have a 1996 Isuzu Trooper with the 3.2L

The engine light has been on for quite a while, I pulled the codes just recently and the scanner retrieved the following:

"O2 sensor out of range... Bank 1 post CAT" (something to that effect)

Following, I replaced the O2 Sensor on the drivers side after the CAT. About 20 miles down the road (After I cleared the codes) the Engine Light re-illuminated. I will pull the codes again tonight and see if something else is now out of range.

Here's the scenarios I am contemplating.

1. I replaced the incorrect O2 sensor
2. The new o2 sensor is faulty
3. CAT is Bad on Drivers Side ( I have been clearing the codes before smog checks here in CA and it passes)

Please, If anyone has any more input I would appreciate your comments. I need to resolve this problem.

Thanks,

Mike