I had identical problem

I installed 1 downstream CAT O2 sensor [ bankk 1] and CEL light has been off for 3 weeks

1999 GREEN Contour SE SPORT Duratec V6, ATX, SUNROOF, 93,000 Miles, P0420 ON&OFF -replaced DS O2 --did not FIX *New TORQUE CONVERTER @ 53,000-Intermittent P1744 *New DASH at 53,000 and New DASH lifted-now using MOLLEY bolts to pull down