The ignition switch on your Mercury Sable sends an electrical signal to the starter motor, allowing you to start your vehicle. A transponder key prevents unauthorised engine starting. If your ignition switch fails, however, this also means that you'll need to replace the switch with one from the parts department at a Ford dealership that services Mercury automobiles (this shouldn't be a problem since Ford owns Mercury as of January 2010) along with the ignition key. In some instances, you may be able to reprogram the key.
Turn the ignition key to the "II" position on the ignition cylinder.
Slide a 1/8-inch punch pin into the access hole directly underneath the ignition cylinder.
Push in on the punch pin while pulling the ignition switch and cylinder out of the steering column with the key.
Remove the electrical leads from the back of the ignition switch.
Install the new ignition switch. Installation is the reverse of removal.