I had a problem with mine on my Zetec. Mine was a bit easier to replace. For more information on replacing your Bank 1 O2 sensor, go to this thread:
http://www.contour.org/cgi-ubb/ultimatebb.cgi?ubb=get_topic&f=3&t=003805&p= If you just want to know how to replace it, I got this for you too
Bank 1 precat O2 sensor replacement procedure
Tools required:
Ramps
7/8 Stubby combination wrench
24" pry bar
Rubber mallet
7mm socket, etc
Procedure:
Put car up on ramps
From above:
Remove hardware securing coil pack
Slide coil pack toward passenger side of car
Locate and disconnect O2 sensor electrical connector
Push connector and wiring down through exhaust manifold runners, toward the ground
From below:
Locate electrical connector
Thread connector through the box end of the 7/8 stubby wrench
Guide the wrench up the wiring harness to the hex-shaped body of the sensor
Position the wrench at the 11 O'clock position (viewed from below) so that the wrench is pointed up through the manifold runners
From above:
Locate the wrench
Carefully, place the pry bar on the wrench handle and smack firmly, but carefully with the mallet to break the sensor free
From below:
Unscrew and remove the sensor
Install new sensor by hand, snug down with wrench
From above:
Reconnect sensor electrical connector
Reinstall coil pack DON'T FORGET THE GROUND WIRE
Clear CEL with scan tool or by removing fuse(s)
Total time is about 1 ½ hours
Total saving is about $200
