On earlier models (I have an 89 P-U) you have to move a screw on the back of your instrument cluster. There should be a screw hole identified as "KSW-A" and "KSW-B". "A" is below a ground terminal/screw and "B" is over to the right with a piece of masking tape over it. Every 90,000 miles you have to move the screw from one position to the other (if the vehicle runs OK, changing the O2 sensor itself isn't needed) .
Hope this helps. Ðave