Problems with the Theft of a Mustang 96

Hey everyone, how's it going?
I have a '96 Mustang and the theft light is flashing. It's giving me code 16, but I'd like to know how I can check it or if anyone has a diagram of the anti-theft system. The car always starts with a key without a chip, which makes me think the problem might be something else (I'm not sure).
Thanks

in advance for any help.