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If the "Theft' light is flashing when trying to start the vehicle, The car is in what is called "Theft Mode" There could be many reasons why the vehicle went into theft mode. A bad igntion key, bad key transponder, bad theft module or another igniton key on your key chain that may interfer with the Ford key or even something magnetic on your key chain. Unforunally there is no much you the customer can do. More than likely the car will have to be towed to the dealer so they can hook up a computer and determine why the vehicle is in theft mode and remove the theft lock on the vehicle. What you can try is this:
If this does not work, The vehicle will need to be towed to the dealer unfortunally.