The passive anti-theft system on Ford F-150 prevents someone from starting the vehicle with an incorrectly coded key.

The passive anti-theft system arms when you switch the ignition off. It disarms when the ignition is switched on with a correctly coded key.
Note: Do not leave a duplicate coded key in your vehicle. Always take the keys and lock all doors when leaving your vehicle.
Note: The system is not compatible with non-Ford aftermarket remote start systems.