The CEL for 420 or 430 require THREE cycles of your engine [1 cycle = ~20 miles of computer required driving]to activate the CEL light [pulling fuses or disconnect battery does the same thing immediately]. This also works in reverse [3 good cycles of no bad O2 inputs the CEL goes out]

Buy at least one downstream O2 sensor and check codes after resetting CEL [Autozone]

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