It won't crank or it cranks but does not start? For the 1999-2004 model year, PATS disables fuel and spark. This gives a crank with no start.
The alarm will go off after the battery is removed and replaced. It does this by design to prevent someone from defeating the alarm by simply removing the battery. Did you turn off the alarm with the key FOB or simply turning the key to run and then off?
Once the alarm is tripped, there is a 10 minute lock out period.
Are there any LARGE metal rings on the key? These can sometimes interfere with the PATS receiver. Recommend taking everything off of the key and try it.
Try this, turn the key to the "run" position (do not start). The theft light should go out after a 3 second "prove out". Post what the light does if it does not go off.
Now crank the car. What does the theft light do? If it is blinking, then repeated attempts to start will cause the alarm to go off and the 10 minute lock out period to start. So, if the car does not start right away and the theft light is blinking, don't just keep trying to start.
There is a procedure to activate the cluster diagnosis. Review: http://www.allfordmustangs.com/forum...stic-mode.html
Use the cluster diagnosis procedure to retrieve the cluster DTC codes (if any). Post.