From my experience...
A non PATS coded key SHOULD not turn the STARTER...!
If ANTI-THEFT were invoked... you can attain a not start condition which the Owners Manual says must be cleared by the dealer.
A working PATS key should disable the anti theft... by turning the ignition on... ELSE try your other key.
The SecuriLock [PATS]
passive anti-theft system is not compatible with
non-Ford aftermarket remote start systems. Use of these systems may
result in vehicle starting problems and a loss of security protection.
Owners Guide (post-2002-fmt)
USA English (fus)
Ford 2003 Mustang Owner Manual
Ford 2003 Mustang Workshop Manual