To reset the computer on a 2002 Ford Taurus disconnect the negative battery cable. Leave the battery cable disconnected for five to ten minutes. Reconnect the cable and start the car. The computer should be reset.
The system will need to be reset or repaired. To reset a original GM system start by using the remote lock and unlock the doors. Then have a repair shop scan the computer and see if it can be reset. If that does not work it will need to see a GM dealer or qualified repair person.
If you try to start the car it should start then turn off by itself, just leave the key in the run position and the "Theft System" light on the dash will be flashing. Leave the key on for about 10 minutes and the light will stop flashing and go on solid, then the theft system is disabled.
to reset the alarm system for a Mercury Sable when the alarm is causing the car not to start, place the key in the ignition. Turn the key only about halfway and leave it in this spot for 10 minutes before trying to start the car.