P0420

I have a 1997 ford taurus. The check engine light is on in the car. I have recently replaced both downstream o2 sensors and reset the check engine light. It goes back on the next time that the car is driven or started. I have found 2 codes on the car after I hooked up a scan tool to the car. They are P0420 and code P1000. I have looked at the data screen on the scanner and I see no readings in mvs or millivolts for the 2 downstream 02 sensors. I would like to know if the sensors are defective or if the engine computer is bad. Also is it possible that the catalytic converter is not reacting correctly any help is appreciated. Thanx