Oxygen sensor - which one?

My 96 v4 contour failed emissions testing. The fault code is P0133 and it said that the O2 sesor circuit had a slow repsonse. I assume that I need to replace the sensor. Which sensor do I need to replace, the one before or after the catalytic converter? Any help is greatly appreciated.

P-0133 = Upstream Heated Oxygen Sensor circuit Slow Response - Bank No. 1.

upstream is before the cat. All other codes can be found on www.contour.org.

http://www.contour.org/FAQ/FAQ.php?s=probs&displayid=10