FORD 2002 Mustang v.2 - Manual (Page 97)

If successful, the new coded key(s) will start the
vehicle’s engine and the theft indicator will
illuminate for three seconds and then go out.
If not successful, the new coded key(s) will not start
the vehicle’s engine and the theft indicator will flash
on and off and you may repeat steps 1 through 6. If
failure repeats, bring your vehicle to your dealership
to have the new spare key(s) programmed.
Locks and Security
97