Volvo Repair: O2 Sensor problem, exhaust leaks, 2001 volvo s40

Question
I have a 2001 Volvo S40 1.9T.  My check engine light keeps coming on.  The code indicate that the O2 sensor before the Catalyst Converter has a slow response.  I had it changed and the code keeps appearing.  Can you help?

Answer
 Eric, check for vacuum or exhaust leaks first. Then you need a dual channel labscope to look at the wave forms of the front and rear sensors to see if it truly is slow and if the rear is reflecting the same result.
  Typically, there is a rich or lean condition that is putting the sensor to max or minimum voltage and holding it there. I doubt the new sensor is bad. Misfires will cause it read full time rich. They can be caused by bad injector(s) or ignition problems.

Mark