4 sensors?
If memory serves me right there are 4 sensors. 2 located behind the engine just forward of the cats and 2 located behind the cats.
After completing my 120,000 mi service I had to replace the 2 located forward of the cats. I had to drop the wye pipe in order to gain access to the fittings. The electrical connection was disconnected and reconnected using long needle nose pliers. My hands would not fit between the bulkhead and the engine.
Just a thought.
1990 RRC, 3.9L, AUTO "Willow Pond" (Sold)
1996 D1, 4.0L, AUTO "Lucas"
1999 RR, 4.0L, AUTO "Belle" (Sold)