Engine will crank over but not start and flashing theft light

Hello, I'm Danny.

Let's try an anti-theft, security system reset first. Here is a system reset tutorial for you to view:

https://www.2carpros.com/articles/how-to-reset-a-security-system

After reading just scroll the page to find Ford. Let us know what happens and we'll go from there. Hope this this helps and thanks for using 2CarPros.

To add to the good tips, I've run into this issue dozens of times at my dealership. If you can get the codes from the instrument cluster, that would help me narrow the issue down incredibly! But a few things that you can check are the 1. Key- is it damaged in any way? 2. Does the vehicle have an aftermarket alarm system, specifically a remote start system? If it does, remove the column shroud and check for another key or chip taped to the back of the ignition lock cylinder, if not disregard. The most common thing that I find is a bad key though, and verified by the trouble code that I get from the instrument cluster module.